kokkoja-rel/client/js/main.js

// @ts-check
/**
 * The Kokkoja client: the start screen, and the session screen that
 * connects the pen, the page and the other participants.
 */

import {
  deriveSession, formatInvitation, formatInvitationLink, loadIdentity, newIdentity, newSecret,
  parseInvitation, saveIdentity,
} from './crypto.js';
import {
  NotebookWriter, discardNotebook, discardPage, keepCurrentPage, keepIdentity, listKeptPages, listNotebooks, notebooksInBrowser,
  readCurrentPage, readIdentity, readPage, rememberToken, useBrowserStorage,
} from './notebook.js';
import { download, pickImageFile } from './browser_files.js';
import { DEFAULT_NOTEBOOK, NotebookPicker } from './notebook_picker.js';
import { pageFromSvg, pageToSvg } from './page_file.js';
import { hashOf } from './crypto.js';
import { imageResolution } from './image_info.js';
import {
  LEAST_TEXT_POINTS, MAX_IMAGE_BYTES, MOST_TEXT_POINTS, POINT_SIZE, TEXT_FONTS, TEXT_SIZE,
  UNITS_PER_POINT, rounded, strokeBox, strokesTouched, textAt, textsTouched,
} from './page_model.js';
import { HOLD_MS, NOMINAL_PRESSURE, attachPenInput } from './pen_input.js';
import { Renderer } from './renderer.js';
import { LEAST_IMAGE_SIDE, textBounds, rectAt } from './page_model.js';
import { inkExtent } from './stroke_geometry.js';
import {
  SHAPE_KEY, SHAPE_NAMES, draggedPoints, isFilled, isTapped, lineInProgressPoints, movedPoints,
  shapeNamed, straightPoints, vertexPoints,
} from './shapes.js';
import { LINE_HEIGHT, TOP_GAP, layoutText } from './text_layout.js';
import { log, logText, trace, watchLog } from './log.js';
import { Session } from './session.js';
import { NotebookChooser } from './chooser.js';
import { FileChooser } from './file_chooser.js';
import { writePicture } from './files.js';
import { pageToPng } from './page_picture.js';
import { TextEditor } from './text_editor.js';
import { enableTooltips } from './tooltip.js';
import { guardSelection, inField } from './selection_guard.js';
import { QUIET_ZONE, qrModules, qrPath } from './qr_code.js';
import { manualList, manualText } from './manual.js';

/**
 * The colours in the toolbar: black, red, green, blue, white (Andy's
 * teacher, 2026-10-08), with the palette's own values so that the palette
 * can leave them out.
 */
const COLORS = ['#000000', '#e53935', '#43a047', '#1e88e5', '#ffffff'];
/** Everyone begins in black (Andy, 2026-10-03; the host began in red before). */
const DEFAULT_COLOR = COLORS[0];

/** Nominal line widths in page units (0.1 mm); the last is twice the one before (2026-10-08). */
const WIDTHS = [4, 7, 12, 24];

/** Whether the page grows when one's writing reaches an edge: 'no' if not. */
const EXTEND_KEY = 'kokkoja_extend';

/** Radius of the eraser on the screen, in CSS pixels. */
const ERASER_RADIUS = 10;

/** With the text tool or the eraser, a press that moves less than this (3 mm) is a tap. */
const TAP_DISTANCE = 30;
/**
 * Two taps of the eraser within this, both finding nothing to erase, undo
 * the latest gesture (Andy, 2026-10-11).  A tap that erases something is
 * never half of a double tap, so the eraser's own taps are unaffected.
 */
const DOUBLE_TAP_MS = 400;

/**
 * An image file is shared as it is if it is no larger than this, in bytes
 * and in pixels along its longer side.  Otherwise it is made smaller and
 * stored as JPEG, so that it travels quickly and the page file stays small.
 */
const IMAGE_AS_IS_BYTES = 1500000;
const IMAGE_LONGEST_SIDE = 2400;

const NAME_KEY = 'kokkoja_name';
/** Colours the user added from the palette, remembered in this browser. */
const COLORS_KEY = 'kokkoja_colors';

/**
 * The palette behind "+": eight hues in three shades, and greys.  Our own,
 * not the browser's colour dialog, which crashes Firefox on Linux.
 */
const PALETTE = [
  '#e53935', '#fb8c00', '#fdd835', '#43a047', '#00acc1', '#1e88e5', '#8e24aa', '#d81b60',
  '#b71c1c', '#e65100', '#f9a825', '#1b5e20', '#006064', '#0d47a1', '#4a148c', '#880e4f',
  '#ffcdd2', '#ffe0b2', '#fff9c4', '#c8e6c9', '#b2ebf2', '#bbdefb', '#e1bee7', '#f8bbd0',
  '#ffffff', '#bdbdbd', '#757575', '#424242', '#8d6e63', '#5d4037', '#3e2723', '#000000',
];
const NOTEBOOK_KEY = 'kokkoja_notebook';
/** The Shape tool's fill colour, remembered in this browser; 'none' for none. */
const FILL_KEY = 'kokkoja_fill';

/** @typedef {import('./pen_input.js').Tool} Tool */
/** @typedef {import('./shapes.js').Shape} Shape */

/**
 * Where the installed Kokkoja program serves the client, for a guest who
 * opened an invitation link on a web site but has the program too.
 */
const INSTALLED_ADDRESS = 'http://localhost:8760/';

/**
 * How this client was served (`config.json`): by the local program, which
 * keeps the notebooks as files ("files"), or by a web site, where the
 * notebooks are kept in the browser ("browser").  The relay is the one for
 * sessions started here; the site, if any, is a Kokkoja web site that an
 * invitation can be a link to.
 * @typedef {object} Config
 * @property {string} relay
 * @property {'files' | 'browser'} storage
 * @property {string} site
 */
/** @type {Config} */
let config = { relay: '', storage: 'files', site: '' };

/**
 * @param {string} id
 * @returns {HTMLElement}
 */
function element(id) {
  return /** @type {HTMLElement} */ (document.getElementById(id));
}

/**
 * @param {string} id
 * @returns {HTMLInputElement}
 */
function input(id) {
  return /** @type {HTMLInputElement} */ (document.getElementById(id));
}

/**
 * @param {string} id
 * @returns {HTMLSelectElement}
 */
function select(id) {
  return /** @type {HTMLSelectElement} */ (document.getElementById(id));
}

/** The start screen's notebook picker. */
const startPicker = new NotebookPicker(
  select('notebook'), input('notebook_name'),
  (message) => { element('start_message').textContent = message; },
  () => {});

/**
 * Our identity: the signing keys that make us the same participant in
 * every session.  The local program keeps them, so that they survive the
 * browser and are the same from any browser on this computer; the first
 * time, they are made here and given to it.
 * @returns {Promise<import('./crypto.js').Identity>}
 */
async function ownIdentity() {
  const kept = await readIdentity();
  if (kept !== null) return loadIdentity(kept);
  const identity = await newIdentity();
  if (await keepIdentity(await saveIdentity(identity))) return identity;
  // Another tab got there first: use what it kept.
  const other = await readIdentity();
  return other !== null ? loadIdentity(other) : identity;
}

// Start screen.

/** @param {string} [message] */
function showStart(message = '') {
  element('session').hidden = true;
  element('start').hidden = false;
  element('start_message').textContent = message;
  input('name').value = localStorage.getItem(NAME_KEY) ?? '';
  const remembered = localStorage.getItem(NOTEBOOK_KEY) ?? DEFAULT_NOTEBOOK;
  startPicker.fill([], remembered);
  startPicker.current = remembered;
  const pending = invitationInAddress();
  if (pending) input('invitation_entry').value = pending;
  // An invitation is a request to join one session: arriving with one,
  // the screen offers joining alone (Andy, 2026-10-06, after starting a
  // session of his own by mistake on a guest's machine).
  element('start_choice').hidden = Boolean(pending);
  // From a web site, a guest who has the Kokkoja program installed may
  // prefer it, for a notebook of real files.
  const installed = /** @type {HTMLAnchorElement} */ (element('open_installed'));
  installed.hidden = !(pending && notebooksInBrowser());
  installed.href = `${INSTALLED_ADDRESS}#join=${encodeURIComponent(pending)}`;
  showNotebooks(startPicker).catch((error) => {
    element('notebook_note').textContent = String(error.message ?? error);
    element('notebook_note').classList.add('problem');
  });
}

/**
 * Offer the existing notebooks in a picker and say where notebooks are kept.
 * The picker's present choice stays chosen: the listing arrives later
 * than the screen, and a notebook made meanwhile must not be lost.
 * @param {NotebookPicker} picker
 */
async function showNotebooks(picker) {
  const { directory, notebooks } = await listNotebooks();
  const current = picker.value;
  picker.fill(notebooks, current);
  picker.current = current;
  for (const note of document.querySelectorAll('.notebook_directory')) {
    note.textContent = directory ? `Notebooks are kept in ${directory}`
      : 'Notebooks are kept in this browser, on this computer. “Keep page” also downloads the page.';
    note.classList.remove('problem');
  }
}

/**
 * The part of the page address after '#', as named values.  The local
 * program puts the token there; a join link puts the invitation there.
 * Browsers do not send this part of an address to any server.
 */
function addressValues() {
  return new URLSearchParams(location.hash.slice(1));
}

/** How long a confirmation stays on the notice line, and a passing problem. */
const NOTICE_MS = 6000;
const PROBLEM_NOTICE_MS = 12000;
/** A reconnection shorter than this is not announced. */
const CONNECTION_GRACE_MS = 1500;

/**
 * The notice line between the toolbar and the page.  It says nothing
 * while all is well.  A STANDING notice, kept by its source until that
 * source clears it, is a state: not connected, the page not being
 * saved, no page from the others yet.  A PASSING notice is a report of
 * one event, kept for a few seconds: a page kept or exported, a file
 * that was not an image.  Standing notices come before a passing one.
 */
/** @type {Map<string, {text: string, problem: boolean}>} */
const standing = new Map();
let passing = { text: '', problem: false };
/** @type {ReturnType<typeof setTimeout> | undefined} */
let passingTimer;

function showNotice() {
  const line = element('notice');
  const states = [...standing.values()];
  const shown = states.length ? states : [passing];
  line.textContent = shown.map((notice) => notice.text).join('  ');
  line.hidden = line.textContent === '';
  line.classList.toggle('problem', shown.some((notice) => notice.problem));
}

/**
 * @param {string} source
 * @param {string} text  '' clears the source's notice.
 * @param {boolean} [problem]
 */
function standingNotice(source, text, problem = true) {
  if (text) standing.set(source, { text, problem });
  else standing.delete(source);
  showNotice();
}

/**
 * @param {string} text
 * @param {boolean} [problem]
 */
function passingNotice(text, problem = false) {
  passing = { text, problem };
  clearTimeout(passingTimer);
  passingTimer = setTimeout(() => {
    passing = { text: '', problem: false };
    showNotice();
  }, problem ? PROBLEM_NOTICE_MS : NOTICE_MS);
  showNotice();
}

/**
 * The last word on saving, in the Session window.
 * @param {string} text
 * @param {boolean} problem
 */
function showSaveState(text, problem) {
  element('save_status').textContent = text;
  element('save_status').classList.toggle('problem', problem);
}

/**
 * The invitation link as a QR code in the Session window, for the camera
 * of a phone or a tablet (Andy, 2026-10-10: there is no good way to send
 * a link from a Linux computer to an iPad).  None for a bare invitation,
 * which a camera could only show as text.
 * @param {string} link
 */
function showInvitationCode(link) {
  const holder = element('invitation_code');
  holder.replaceChildren();
  holder.hidden = link === '';
  if (link === '') return;
  const modules = qrModules(link);
  const span = modules.length + 2 * QUIET_ZONE;
  const svg = document.createElementNS('http://www.w3.org/2000/svg', 'svg');
  svg.setAttribute('viewBox', `${-QUIET_ZONE} ${-QUIET_ZONE} ${span} ${span}`);
  svg.setAttribute('shape-rendering', 'crispEdges');
  svg.setAttribute('role', 'img');
  svg.setAttribute('aria-label', 'The invitation as a QR code');
  const path = document.createElementNS('http://www.w3.org/2000/svg', 'path');
  path.setAttribute('d', qrPath(modules));
  path.setAttribute('fill', '#000');
  svg.append(path);
  holder.append(svg);
}

/** @returns {string}  The invitation in the page address, or ''. */
function invitationInAddress() {
  return addressValues().get('join') ?? '';
}

/**
 * Set the page address for a session, or for none.
 * @param {string} [invitation]
 */
function setAddress(invitation) {
  history.replaceState(null, '', invitation
    ? `#join=${encodeURIComponent(invitation)}` : location.pathname);
}

/** @returns {string | null}  The name entered, or null after complaining. */
function enteredName() {
  const name = input('name').value.trim();
  if (!name) {
    element('start_message').textContent = 'Please enter your name.';
    input('name').focus();
    return null;
  }
  localStorage.setItem(NAME_KEY, name);
  return name;
}

/** @returns {string}  The notebook chosen. */
function enteredNotebook() {
  const notebook = startPicker.value || DEFAULT_NOTEBOOK;
  localStorage.setItem(NOTEBOOK_KEY, notebook);
  return notebook;
}

async function startSession() {
  const name = enteredName();
  if (!name) return;
  const identity = await ownIdentity();
  const secret = newSecret();
  const invitation = formatInvitation({ relay: config.relay, secret, hostId: identity.id });
  await enterSession(invitation, name, enteredNotebook());
}

async function joinSession() {
  const name = enteredName();
  if (!name) return;
  const invitation = input('invitation_entry').value.trim();
  if (!parseInvitation(invitation)) {
    element('start_message').textContent = 'That is not a Kokkoja invitation.';
    return;
  }
  await enterSession(invitation, name, enteredNotebook());
}

// Session screen.

/**
 * Prepare an image file to be put on the page.  The image's natural size
 * comes from the resolution the file records, if any, and otherwise from
 * taking a pixel as 0.1 mm (Andy's decision, 2026-10-03); where it goes on
 * the page is decided when it is added.  An image of more pixels or bytes
 * than is useful is made smaller here, which does not change its natural
 * size.
 * @param {Blob} file
 * @returns {Promise<Parameters<Session['addImage']>[0]>}
 */
async function prepareImage(file) {
  const original = new Uint8Array(await file.arrayBuffer());
  const bitmap = await createImageBitmap(file);
  const dpi = imageResolution(original);
  const unitsPerPixel = dpi === null ? 1 : 254 / dpi;
  let blob = file;
  const longest = Math.max(bitmap.width, bitmap.height);
  const asIs = (file.type === 'image/png' || file.type === 'image/jpeg') &&
    file.size <= IMAGE_AS_IS_BYTES && longest <= IMAGE_LONGEST_SIDE;
  if (!asIs) {
    const scale = Math.min(1, IMAGE_LONGEST_SIDE / longest);
    const canvas = document.createElement('canvas');
    canvas.width = Math.round(bitmap.width * scale);
    canvas.height = Math.round(bitmap.height * scale);
    const context = /** @type {CanvasRenderingContext2D} */ (canvas.getContext('2d'));
    context.fillStyle = '#ffffff';
    context.fillRect(0, 0, canvas.width, canvas.height);
    context.drawImage(bitmap, 0, 0, canvas.width, canvas.height);
    for (const quality of [0.9, 0.75, 0.5]) {
      blob = await new Promise((resolve, reject) => canvas.toBlob(
        (result) => (result ? resolve(result) : reject(new Error('The image could not be prepared.'))),
        'image/jpeg', quality));
      if (blob.size <= MAX_IMAGE_BYTES) break;
    }
  }
  if (blob.size > MAX_IMAGE_BYTES) throw new Error('The image is too large.');
  const data = blob === file ? original : new Uint8Array(await blob.arrayBuffer());
  const type = blob.type === 'image/png' ? 'image/png' : 'image/jpeg';
  return {
    type, data, hash: await hashOf(data),
    naturalWidth: bitmap.width * unitsPerPixel, naturalHeight: bitmap.height * unitsPerPixel,
  };
}

/**
 * The page to begin a session with, from the notebook's current page file.
 * If that file holds a page of this very session (the browser page was
 * reloaded, or we left and came back), the session continues from it.  If
 * it holds a page of another session, that page is kept first, because the
 * file is about to be given over to this session.
 * @param {string} notebook
 * @param {string} roomId
 * @returns {Promise<import('./page_model.js').Page | undefined>}
 */
async function pageToContinue(notebook, roomId) {
  const text = await readCurrentPage(notebook);
  if (text === null) return undefined;
  const saved = pageFromSvg(text);
  if (saved && saved.session === roomId) return saved.page;
  await keepCurrentPage(notebook);
  return undefined;
}

/** True once this browser page has entered a session. */
let entered = false;

/**
 * Where a browser tab remembers the notebook it chose for a session.
 * @param {string} roomId
 */
function notebookKeyFor(roomId) {
  return `kokkoja_notebook_${roomId}`;
}

/**
 * @param {string} invitationText
 * @param {string} name
 * @param {string} notebook
 */
async function enterSession(invitationText, name, notebook) {
  const invitation = parseInvitation(invitationText);
  if (!invitation) {
    showStart('That is not a Kokkoja invitation.');
    return;
  }
  if (entered) return;
  entered = true;
  const { roomId, key } = await deriveSession(invitation.secret);
  const identity = await ownIdentity();
  // A browser tab stays with the notebook it chose for a session.
  const notebookKey = notebookKeyFor(roomId);
  notebook = sessionStorage.getItem(notebookKey) ?? notebook;
  sessionStorage.setItem(notebookKey, notebook);
  const savedPage = await pageToContinue(notebook, roomId);
  setAddress(invitationText);

  element('start').hidden = true;
  element('session').hidden = false;
  // The invitation is shown as a link to the web site when there is one,
  // so that a guest can open it and be in the session with nothing
  // installed.  The link carries the invitation itself.
  const plainInvitation = formatInvitation(invitation);
  input('invitation').value = config.site ? formatInvitationLink(config.site, plainInvitation) : plainInvitation;
  showInvitationCode(config.site ? input('invitation').value : '');

  let tool = /** @type {Tool} */ ('pen');
  /** The Shape tool's current shape, remembered in this browser (Andy, 2026-10-14). */
  let shape = shapeNamed(localStorage.getItem(SHAPE_KEY));
  /** The colour a shape is filled with, or null (Andy, 2026-10-14); separate from the line's colour. */
  let fill = /^#[0-9a-f]{6}$/.exec(localStorage.getItem(FILL_KEY) ?? '')?.[0] ?? null;
  let color = DEFAULT_COLOR;
  let width = WIDTHS[1];
  let textSize = TEXT_SIZE;
  let textFont = /** @type {import('./page_model.js').TextFont} */ ('sans-serif');

  /** @type {Renderer} */
  let renderer;
  let layerOrder = '';

  function syncLayers() {
    const order = session.layerOrder();
    if (order.join(' ') === layerOrder) return;
    layerOrder = order.join(' ');
    renderer.setLayerOrder(order);
  }

  /**
   * @param {import('./page_model.js').Page} page
   * @returns {string}  The page file.
   */
  const fileOf = (page) => pageToSvg(page, { session: roomId, hostId: invitation.hostId });

  function showPageNumber() {
    const waiting = session.page.number === 0;
    element('page_number').textContent = waiting ? 'No page yet' : `Page ${session.page.number}`;
    standingNotice('page', waiting ? 'Waiting for the others’ page; New starts one' : '', false);
  }
  /** @type {ReturnType<typeof setTimeout> | undefined} */
  let connectionNoticeTimer;

  const session = new Session(
    {
      relay: invitation.relay, roomId, key, identity, hostId: invitation.hostId, name,
      page: savedPage,
    },
    {
      onLayerReplaced: (layerId) => {
        syncLayers();
        renderer.redrawLayer(layerId);
        if (layerId === session.identity.id) refreshArrangeCues();
        writer.changed();
      },
      onStrokeProgress: (layerId, stroke) => {
        syncLayers();
        renderer.strokeProgress(layerId, stroke);
        if (stroke.done) {
          if (stroke.shape && layerId === session.identity.id) refreshArrangeCues();
          writer.changed();
        }
      },
      onTextChanged: (layerId) => {
        syncLayers();
        renderer.redrawLayer(layerId);
        if (layerId === session.identity.id) refreshArrangeCues();
        writer.changed();
      },
      onPageReplaced: (oldPage, newPage) => {
        log(`Page ${newPage.number} (${newPage.images.length} images) replaces page ${oldPage.number}`);
        // A block being typed stays on the page being left, where every
        // keystroke already put it; finishing the edit now would move it
        // to the new page (seen by Andy, 2026-10-06).  An empty one is
        // dropped from the old page before that page is kept.
        const abandoned = textEditor?.discard() ?? null;
        if (abandoned && abandoned.text.trim() === '') {
          oldPage.layers.get(session.identity.id)?.texts.delete(abandoned.id);
        }
        // A line being made likewise stays on the page being left (the
        // session has ended its stroke).
        lineInProgress = null;
        renderer.hideShapePreview();
        // Nothing is lost: the page being left is kept in the notebook.
        writer.pageReplaced(oldPage.number > 0 ? fileOf(oldPage) : null);
        renderer.setPage(newPage);
        layerOrder = '';
        syncLayers();
        showParticipants(session, renderer);
        showPageNumber();
        refreshArrangeCues();
      },
      onImagesChanged: () => {
        log(`Page ${session.page.number}: ${session.page.images.length} images, ${session.page.hides.length} hides`);
        renderer.showImages();
        renderer.drawHides();
        refreshArrangeCues();
        writer.writeNow();
      },
      onPageResized: () => {
        // The view keeps its scale; the page extends beyond the window.
        renderer.resize();
        textEditor?.layout();
        writer.changed();
      },
      onParticipantsChanged: () => {
        syncLayers();
        showParticipants(session, renderer);
      },
      onUndoChanged: () => {
        /** @type {HTMLButtonElement} */ (element('undo')).disabled = !session.canUndo();
      },
      onStatus: (status, detail) => {
        const text = { connecting: 'Connecting…', connected: 'Connected', failed: detail ?? 'Disconnected' };
        log(`Relay: ${text[status]}`);
        element('status').textContent = text[status];
        element('status').classList.toggle('problem', status !== 'connected');
        // The notice line says nothing while connected, nothing during a
        // short reconnection either, and stays up while a longer one lasts.
        // Every attempt reports 'connecting' again, so the grace runs from
        // the first of them.
        if (status === 'connecting') {
          if (connectionNoticeTimer === undefined && !standing.has('connection')) {
            connectionNoticeTimer = setTimeout(() => {
              connectionNoticeTimer = undefined;
              standingNotice('connection', 'Not connected. Reconnecting…');
            }, CONNECTION_GRACE_MS);
          }
          return;
        }
        clearTimeout(connectionNoticeTimer);
        connectionNoticeTimer = undefined;
        standingNotice('connection', status === 'connected' ? '' : text.failed);
      },
    });

  const writer = new NotebookWriter(
    notebook,
    () => (session.page.number > 0 ? fileOf(session.page) : null),
    {
      onSaved: (time) => {
        showSaveState(`Saved at ${time}`, false);
        standingNotice('saving', '');
      },
      onProblem: (message) => {
        showSaveState(message, true);
        standingNotice('saving', message);
      },
      onKept: (message) => passingNotice(message),
    });
  showSaveState(savedPage ? 'Continued from the notebook' : '', false);

  const pageElement = element('page');
  renderer = new Renderer(pageElement, session.page);
  /** @type {TextEditor | null} */
  let textEditor = null;
  syncLayers();
  for (const layerId of session.page.layers.keys()) renderer.redrawLayer(layerId);
  showParticipants(session, renderer);
  showPageNumber();
  new ResizeObserver(() => {
    renderer.resize();
    textEditor?.layout();
  }).observe(/** @type {Element} */ (pageElement.parentElement));

  // The toolbar, when the window is too narrow for one row, centres its
  // rows rather than holding the two ends (Andy, 2026-10-09).  Wrapping
  // is told from the toolbar's height against one row's.
  const toolbar = /** @type {HTMLElement} */ (document.querySelector('.toolbar'));
  const watchToolbar = () => {
    // One row is a control's height plus the toolbar's padding; two rows
    // are nearly twice that.
    const row = /** @type {HTMLElement} */ (toolbar.firstElementChild).offsetHeight;
    toolbar.classList.toggle('wrapped', toolbar.clientHeight > row * 1.9);
  };
  new ResizeObserver(watchToolbar).observe(toolbar);
  watchToolbar();
  // The drawing group and the settings (colours, widths, font and size)
  // keep the width of their widest state, so that Eraser, Hide and Undo,
  // which stand after the settings since 2026-10-11, do not jump when a
  // tool shows fewer settings (the drawing group's floor also kept the
  // middle group still while there was one, until 2026-10-14).  The
  // floor rises if a group ever grows, as when a colour is added.
  for (const part of [element('drawing'), element('settings')]) {
    let widest = 0;
    new ResizeObserver(() => {
      const width = part.getBoundingClientRect().width;
      if (width > widest) {
        widest = width;
        part.style.minWidth = `${width}px`;
      }
    }).observe(part);
  }

  textEditor = new TextEditor(pageElement, renderer, {
    setText: (block) => session.setText(block),
    removeText: (id) => session.removeTexts([id]),
    beginEdit: (id) => session.beginTextEdit(id, 'editor'),
    endEdit: (id) => session.endTextEdit(id, 'editor'),
    // A hold on the field being typed in moves its block, as on the page.
    beginHold: (clientX, clientY) => penTarget.beginHold?.(clientX, clientY) ?? false,
    holdMove: (clientX, clientY) => penTarget.holdMove?.(clientX, clientY),
    holdEnd: () => penTarget.holdEnd?.(),
  });

  // Undo: one's own latest gesture, then the one before (Andy,
  // 2026-10-11): the button, Ctrl+Z (Command+Z on a Mac) outside a field,
  // and two quick taps of the eraser on empty paper (eraseEnded below).
  // A block being typed, or a line being made, is finished first, so that
  // the undo is of it.
  const undo = () => {
    textEditor?.stop();
    finishLine();
    const what = session.undo();
    if (what) log(`Undid ${what}`);
  };
  element('undo').addEventListener('click', undo);
  document.addEventListener('keydown', (event) => {
    if ((event.ctrlKey || event.metaKey) && !event.shiftKey && !event.altKey &&
        event.key.toLowerCase() === 'z' && !inField()) {
      event.preventDefault();
      undo();
    }
    // A line being made: Escape finishes it, Backspace takes back its
    // last point.
    if (lineInProgress && !inField() && (event.key === 'Escape' || event.key === 'Backspace')) {
      event.preventDefault();
      if (event.key === 'Escape') finishLine();
      else removeLastVertex();
    }
  });

  // Whether one's own writing may grow the page (the "Extend" box).
  let extend = localStorage.getItem(EXTEND_KEY) !== 'no';
  const extendBox = input('extend');
  extendBox.checked = extend;
  extendBox.addEventListener('change', () => {
    extend = extendBox.checked;
    localStorage.setItem(EXTEND_KEY, extend ? 'yes' : 'no');
  });
  /**
   * With "Extend" off, points that would make the page grow are dropped:
   * the stroke stops short of the edge and resumes when the pen is back
   * inside.  Nothing is stored beyond the page.
   * @param {number[]} points
   * @returns {number[]}
   */
  const withinPage = (points) => {
    if (extend) return points;
    const page = session.page;
    const kept = [];
    for (let i = 0; i < points.length; i += 3) {
      if (page.holdsWriting(points[i], points[i + 1], width)) kept.push(points[i], points[i + 1], points[i + 2]);
    }
    return kept;
  };

  let reportedDevice = '';

  /**
   * What a hold of the pen is handling: a Hide rectangle, a text block or
   * a shape of ours, or an image; which; whether a corner is being
   * dragged (and then which, with the opposite one fixed); and how the
   * pen's position maps to the rectangle.  Images keep their shape when
   * scaled; the others resize freely (Andy, 2026-10-08).
   * @typedef {object} Held
   * @property {'hide' | 'text' | 'shape' | 'image'} kind
   * @property {number} index        Of the Hide rectangle or image.
   * @property {string} id           Of the text block or the shape's stroke.
   * @property {[number, number] | null} corner
   * @property {[number, number]} fixed
   * @property {number} aspect
   * @property {number} offsetX
   * @property {number} offsetY
   */
  /** @type {Held | null} */
  let held = null;
  /** When the eraser last tapped empty paper, by performance.now(); 0 if not lately. */
  let lastEmptyTap = 0;
  /** @type {import('./page_model.js').Stroke | null}  The stroke a hold began as. */
  let strokeInProgress = null;

  /**
   * The rectangle a hold has moved or resized to, from the pen's position.
   * @param {Held} h
   * @param {import('./page_model.js').Rectangle} now  The rectangle as it is.
   * @param {number} x
   * @param {number} y
   * @param {boolean} keepShape
   * @returns {import('./page_model.js').Rectangle}
   */
  function heldRect(h, now, x, y, keepShape) {
    if (!h.corner) return { left: x - h.offsetX, top: y - h.offsetY, width: now.width, height: now.height };
    const [fx, fy] = h.fixed;
    const dirX = h.corner[0] ? 1 : -1;
    const dirY = h.corner[1] ? 1 : -1;
    let w = Math.max((x - fx) * dirX, LEAST_IMAGE_SIDE);
    let hgt = Math.max((y - fy) * dirY, LEAST_IMAGE_SIDE);
    if (keepShape) {
      // Along the diagonal from the fixed corner, so the aspect ratio holds.
      w = Math.max((x - fx) * dirX, (y - fy) * dirY * h.aspect, LEAST_IMAGE_SIDE, LEAST_IMAGE_SIDE * h.aspect);
      hgt = w / h.aspect;
    }
    return { left: dirX > 0 ? fx : fx - w, top: dirY > 0 ? fy : fy - hgt, width: w, height: hgt };
  }

  /**
   * @param {Held['kind']} kind
   * @param {number} index
   * @param {string} id
   * @param {import('./page_model.js').Rectangle} r
   * @param {[number, number] | null} corner
   * @param {number} x
   * @param {number} y
   * @returns {Held}
   */
  const hold = (kind, index, id, r, corner, x, y) => ({
    kind, index, id, corner,
    fixed: corner ? [r.left + (1 - corner[0]) * r.width, r.top + (1 - corner[1]) * r.height] : [0, 0],
    aspect: r.width / r.height, offsetX: x - r.left, offsetY: y - r.top,
  });

  /**
   * @param {Held} h
   * @returns {import('./page_model.js').Rectangle | null}
   */
  function currentHeldRect(h) {
    const page = session.page;
    if (h.kind === 'hide') {
      const hide = page.hides[h.index];
      return hide ? { left: hide.x, top: hide.y, width: hide.width, height: hide.height } : null;
    }
    if (h.kind === 'image') {
      const image = page.images[h.index];
      return image ? { left: image.x, top: image.y, width: image.width, height: image.height } : null;
    }
    if (h.kind === 'shape') {
      const stroke = session.ownLayer.strokes.get(h.id);
      return stroke ? strokeBox(stroke) : null;
    }
    const block = session.ownLayer.texts.get(h.id);
    return block ? textBounds(block) : null;
  }

  /**
   * The grip around the held thing: for a stroke, around its ink rather
   * than its points (Andy, 2026-10-14).
   * @param {Held} h
   */
  function showHeldGrip(h) {
    if (h.kind === 'shape') {
      const stroke = session.ownLayer.strokes.get(h.id);
      renderer.showGrip(stroke ? inkExtent(stroke) : null, h.corner, true);
    } else {
      renderer.showGrip(currentHeldRect(h), h.corner, heldHasCorners(h));
    }
  }

  /**
   * @param {Held} h
   * @returns {boolean}  Whether the held thing has corners to resize by:
   *     everything but free text.
   */
  const heldHasCorners = (h) => h.kind !== 'text' || session.ownLayer.texts.get(h.id)?.height !== null;

  /** @returns {import('./page_model.js').Stroke[]}  Our shapes, in drawing order. */
  const ownShapes = () => [...session.ownLayer.strokes.values()].filter((stroke) => stroke.shape !== null && stroke.done);

  /**
   * With the Arrange tool chosen, mark everything a press can move or
   * resize with a blue box and corners (Andy, 2026-10-14): the images
   * and Hides, one's own text blocks and shapes; free text, which has
   * no width to resize, gets the box without the corners.  The one being
   * held is left out, the green grip standing in its place until the
   * pointer lifts.  Called whenever the images, the Hides, one's own
   * text or shapes, or the hold may have changed.
   */
  function refreshArrangeCues() {
    if (tool !== 'arrange') {
      renderer.setArrangeCues([]);
      return;
    }
    /** @type {{rect: import('./page_model.js').Rectangle, kind: 'image' | 'text' | 'hide' | 'shape' | 'stroke', corners: boolean}[]} */
    const cues = [];
    const page = session.page;
    page.images.forEach((im, index) => {
      if (held?.kind === 'image' && held.index === index) return;
      cues.push({ kind: 'image', rect: { left: im.x, top: im.y, width: im.width, height: im.height }, corners: true });
    });
    page.hides.forEach((h, index) => {
      if (held?.kind === 'hide' && held.index === index) return;
      cues.push({ kind: 'hide', rect: { left: h.x, top: h.y, width: h.width, height: h.height }, corners: true });
    });
    for (const block of session.ownLayer.texts.values()) {
      if (held?.kind === 'text' && held.id === block.id) continue;
      cues.push({ kind: 'text', rect: textBounds(block), corners: block.height !== null });
    }
    for (const stroke of ownShapes()) {
      if (held?.kind === 'shape' && held.id === stroke.id) continue;
      const rect = inkExtent(stroke);
      if (rect) cues.push({ kind: 'shape', rect, corners: true });
    }
    // The handwritten stroke under the hovering pointer, if any.
    const hovered = hoveredStroke && session.ownLayer.strokes.get(hoveredStroke);
    if (hovered && !(held?.kind === 'shape' && held.id === hovered.id)) {
      const rect = inkExtent(hovered);
      if (rect) cues.push({ kind: 'stroke', rect, corners: true });
    }
    renderer.setArrangeCues(cues);
  }

  /**
   * Handwriting under Arrange (Andy, 2026-10-14): a page may hold hundreds
   * of strokes, so they are not all boxed at once; the one whose ink the
   * pointer hovers over gets the box, kept while the pointer stays within
   * it (so that its corners can be reached), and a press takes hold of a
   * stroke by its ink, or by the shown box's corner, under Arrange only:
   * the hold with another tool leaves handwriting alone, since resting
   * the pen on one's own writing is part of writing.
   * @type {string | null}  The id of the stroke whose box is shown.
   */
  let hoveredStroke = null;

  /**
   * The frontmost handwritten stroke of ours (not a shape, which has its
   * cue always) within the eraser's reach of the point, by its ink.
   * @param {number} x
   * @param {number} y
   * @returns {import('./page_model.js').Stroke | null}
   */
  function handwritingAt(x, y) {
    const layer = session.ownLayer;
    const ids = strokesTouched(layer, x, y, x, y, renderer.pageLength(ERASER_RADIUS));
    for (let i = ids.length - 1; i >= 0; i--) {
      const stroke = layer.strokes.get(ids[i]);
      if (stroke && stroke.done && stroke.shape === null) return stroke;
    }
    return null;
  }

  /**
   * Under Arrange, the pointer hovering at this point: the stroke under
   * it gets the box, or the shown box stays while the pointer is within
   * it or near its corners.
   * @param {number} x
   * @param {number} y
   */
  function hoverHandwriting(x, y) {
    const shown = hoveredStroke && session.ownLayer.strokes.get(hoveredStroke);
    const box = shown ? inkExtent(shown) : null;
    const reach = renderer.cornerReach();
    const within = box && x >= box.left - reach && x <= box.left + box.width + reach && y >= box.top - reach && y <= box.top + box.height + reach;
    const under = handwritingAt(x, y);
    const next = under ? under.id : within ? hoveredStroke : null;
    if (next === hoveredStroke) return;
    hoveredStroke = next;
    refreshArrangeCues();
  }

  // The Shape tool (Andy, 2026-10-14; shapes.js): a rectangle, an ellipse
  // or an arrow is dragged open and becomes one stroke when the pointer
  // lifts, a preview showing meanwhile; a line is made of taps, one
  // vertex each, its stroke growing for everyone with each tap, a rubber
  // band from the last vertex following a hovering pointer, until a tap
  // on the last vertex (finished open) or on the first (closed), Escape,
  // or a change of tool or shape.

  /** @type {{stroke: import('./page_model.js').Stroke, vertices: [number, number][]} | null} */
  let lineInProgress = null;

  /** The line being made is complete. */
  function finishLine() {
    const line = lineInProgress;
    if (!line) return;
    lineInProgress = null;
    renderer.hideShapePreview();
    if (session.currentStroke === line.stroke) session.endStroke();
    log(`Line of ${line.vertices.length} points`);
  }

  /**
   * Backspace while a line is made: its last vertex goes.  A stroke in
   * progress cannot lose points by any message, so the stroke is erased
   * and begun again with the vertices left.
   */
  function removeLastVertex() {
    const line = lineInProgress;
    if (!line) return;
    line.vertices.pop();
    if (session.currentStroke === line.stroke) session.endStroke(false);
    session.erase([line.stroke.id]);
    if (line.vertices.length === 0) {
      lineInProgress = null;
      renderer.hideShapePreview();
      return;
    }
    line.stroke = session.beginStroke(color, width, { fill, kind: 'line' });
    session.addPoints(lineInProgressPoints(line.vertices));
  }

  /**
   * Whether a shape's points all lie where the page need not grow, or
   * Extend is on.  With Extend off a shape is drawn whole or not at all,
   * since dropping its points beyond the edge, as a stroke's are, would
   * deform it.
   * @param {number[]} points
   */
  function pageHolds(points) {
    if (extend) return true;
    for (let i = 0; i < points.length; i += POINT_SIZE) {
      if (!session.page.holdsWriting(points[i], points[i + 1], width)) return false;
    }
    return true;
  }

  /**
   * The pointer went down at (x0, y0) and up at (x1, y1) with the Shape
   * tool.
   * @param {number} x0
   * @param {number} y0
   * @param {number} x1
   * @param {number} y1
   */
  function placeShape(x0, y0, x1, y1) {
    renderer.hideShapePreview();
    const dragged = Math.hypot(x1 - x0, y1 - y0) >= TAP_DISTANCE;
    /** @param {number} v */
    const r = (v) => rounded(v, 10);
    if (!isTapped(shape)) {
      if (!dragged) return;
      const points = draggedPoints(shape, r(x0), r(y0), r(x1), r(y1), width);
      if (!pageHolds(points)) {
        trace('shape refused: it would reach past the page with Extend off');
        return;
      }
      textEditor?.stop();
      session.beginStroke(color, width, { filled: isFilled(shape), fill: isFilled(shape) ? null : fill, kind: shape });
      session.addPoints(points);
      session.endStroke();
      log(`${SHAPE_NAMES[shape]} drawn${fill && !isFilled(shape) ? `, filled ${fill}` : ''}`);
      return;
    }
    // A line: each press and release gives a vertex, the release point.
    const vx = r(x1);
    const vy = r(y1);
    if (lineInProgress && session.currentStroke !== lineInProgress.stroke) lineInProgress = null;
    const line = lineInProgress;
    if (line) {
      const reach = renderer.cornerReach();
      const [fx, fy] = line.vertices[0];
      const [lx, ly] = line.vertices[line.vertices.length - 1];
      if (line.vertices.length >= 2 && Math.hypot(vx - fx, vy - fy) <= reach) {
        session.addPoints([fx, fy, NOMINAL_PRESSURE]);     // Closed: back to the first point.
        finishLine();
      } else if (Math.hypot(vx - lx, vy - ly) <= reach) {
        finishLine();
      } else if (pageHolds([vx, vy, NOMINAL_PRESSURE])) {
        line.vertices.push([vx, vy]);
        session.addPoints(vertexPoints(vx, vy));
      } else {
        trace('line point refused: beyond the page with Extend off');
      }
      return;
    }
    // The first vertex; a drag gives the first two.
    /** @type {[number, number][]} */
    const vertices = dragged ? [[r(x0), r(y0)], [vx, vy]] : [[vx, vy]];
    const points = lineInProgressPoints(vertices);
    if (!pageHolds(points)) {
      trace('line refused: beyond the page with Extend off');
      return;
    }
    textEditor?.stop();
    const stroke = session.beginStroke(color, width, { fill, kind: 'line' });
    lineInProgress = { stroke, vertices };
    session.addPoints(points);
  }

  /**
   * The implied rectangle under a point, for the hover cue (Andy,
   * 2026-10-14: text rectangles and Hides show their corners only then,
   * like free text): the frontmost Hide, else the frontmost text block
   * among the layers shown, with its kind, which decides the cue's form.
   * @param {number} x
   * @param {number} y
   * @returns {{rect: import('./page_model.js').Rectangle, kind: 'hide' | 'rect' | 'free'} | null}
   */
  function impliedRectAt(x, y) {
    /** @param {import('./page_model.js').Rectangle} b */
    const holds = (b) => x >= b.left && x <= b.left + b.width && y >= b.top && y <= b.top + b.height;
    for (const h of [...session.page.hides].reverse()) {
      const b = { left: h.x, top: h.y, width: h.width, height: h.height };
      if (holds(b)) return { rect: b, kind: 'hide' };
    }
    for (const id of session.layerOrder().reverse()) {
      const layer = session.page.layers.get(id);
      if (!layer || renderer.hidden.has(id)) continue;
      for (const block of [...layer.texts.values()].reverse()) {
        const b = textBounds(block);
        if (holds(b)) return { rect: b, kind: block.height === null ? 'free' : 'rect' };
      }
    }
    return null;
  }

  /**
   * What a press of the eraser at this point would remove, for the cue
   * shown while the pointer hovers (Andy, 2026-10-14): the rectangles
   * around the strokes and text blocks of ours within the eraser's reach,
   * whole, as the eraser takes them; or, where there is none, the Hide or
   * image a tap would remove, by the same rules as eraseEnded.
   * @param {number} x
   * @param {number} y
   * @returns {import('./page_model.js').Rectangle[]}
   */
  function eraseTargetsAt(x, y) {
    const layer = session.ownLayer;
    const radius = renderer.pageLength(ERASER_RADIUS);
    /** @type {import('./page_model.js').Rectangle[]} */
    const rects = [];
    for (const id of strokesTouched(layer, x, y, x, y, radius)) {
      const stroke = layer.strokes.get(id);
      const rect = stroke && inkExtent(stroke);
      if (rect) rects.push(rect);
    }
    for (const id of textsTouched(layer, x, y, x, y, radius)) {
      const block = layer.texts.get(id);
      if (block) rects.push(textBounds(block));
    }
    if (rects.length) return rects;
    const page = session.page;
    const hide = page.hideAt(x, y, 0);
    if (hide) {
      const h = page.hides[hide.index];
      return [{ left: h.x, top: h.y, width: h.width, height: h.height }];
    }
    const found = page.imageAt(x, y, 0);
    const image = found && page.images[found.index];
    if (image) {
      return [{ left: image.x, top: image.y, width: image.width, height: image.height }];
    }
    return [];
  }

  /** @type {import('./pen_input.js').PenTarget} */
  const penTarget = {
    toPage: (clientX, clientY) => renderer.pageFromClient(clientX, clientY),
    tool: () => tool,
    canWrite: () => session.page.number > 0,
    beginStroke: () => {
      textEditor?.stop();
      strokeInProgress = session.beginStroke(color, width);
    },
    addPoints: (points) => {
      const inside = withinPage(points);
      if (inside.length) session.addPoints(inside);
    },
    endStroke: () => {
      strokeInProgress = null;
      session.endStroke();
    },
    beginZoom: (clientX, clientY) => renderer.beginZoomDrag(clientX, clientY),
    zoomTo: (pixelsUp) => renderer.zoomDragTo(pixelsUp),
    endZoom: () => renderer.endZoomDrag(),
    // A hold moves what is under the pen, or resizes it from a corner: a
    // Hide rectangle, an image, or a text block of our own.  The dot
    // drawn meanwhile goes.
    beginHold: (clientX, clientY) => {
      const [x, y] = renderer.pageFromClient(clientX, clientY);
      const reach = renderer.cornerReach();
      const page = session.page;
      /** @type {Held | null} */
      let found = null;
      const hide = page.hideAt(x, y, reach);
      if (hide) {
        const h = page.hides[hide.index];
        found = hold('hide', hide.index, h.id, { left: h.x, top: h.y, width: h.width, height: h.height }, hide.corner, x, y);
        session.beginHideEdit(h.id);
      } else {
        const blocks = [...session.ownLayer.texts.values()];
        const at = rectAt(blocks.map(textBounds), x, y, reach);
        // Handwriting, under Arrange only: the shown box's corner, or a
        // stroke by its ink.
        /** @type {{stroke: import('./page_model.js').Stroke, corner: [number, number] | null} | null} */
        let writing = null;
        if (!at && tool === 'arrange') {
          const shown = hoveredStroke && session.ownLayer.strokes.get(hoveredStroke);
          const shownBox = shown ? inkExtent(shown) : null;
          const cornerAt = shownBox ? rectAt([shownBox], x, y, reach) : null;
          if (shown && cornerAt?.corner) writing = { stroke: shown, corner: cornerAt.corner };
          else {
            const under = handwritingAt(x, y);
            if (under) writing = { stroke: under, corner: null };
          }
        }
        const shapes = at || writing ? [] : ownShapes();
        const shapeAt = at || writing ? null : rectAt(shapes.map((s) => strokeBox(s) ?? { left: 0, top: 0, width: 0, height: 0 }), x, y, reach);
        if (at) {
          const block = blocks[at.index];
          // Free text has no corners to resize by.
          const corner = block.height === null ? null : at.corner;
          found = hold('text', at.index, block.id, textBounds(block), corner, x, y);
          session.beginTextEdit(block.id, 'hold');
        } else if (writing) {
          const box = /** @type {import('./page_model.js').Rectangle} */ (strokeBox(writing.stroke));
          if (session.beginShapeEdit(writing.stroke.id)) found = hold('shape', 0, writing.stroke.id, box, writing.corner, x, y);
        } else if (shapeAt) {
          // A shape of ours (Andy, 2026-10-14): moved, or resized from a
          // corner, by the rectangle around its points.
          const stroke = shapes[shapeAt.index];
          const box = /** @type {import('./page_model.js').Rectangle} */ (strokeBox(stroke));
          if (session.beginShapeEdit(stroke.id)) found = hold('shape', shapeAt.index, stroke.id, box, shapeAt.corner, x, y);
        } else {
          const image = page.imageAt(x, y, reach);
          if (image) {
            const index = session.raiseImage(image.index);
            const im = page.images[index];
            found = hold('image', index, '', { left: im.x, top: im.y, width: im.width, height: im.height }, image.corner, x, y);
          }
        }
      }
      if (!found) return false;
      if (strokeInProgress) {
        const begun = strokeInProgress;
        strokeInProgress = null;
        session.endStroke(false);
        session.erase([begun.id]);
      }
      textEditor?.hidePreview();
      held = found;
      showHeldGrip(found);
      refreshArrangeCues();    // The held thing's blue cue gives way to the grip.
      log(`${found.kind} held${found.corner ? ' at a corner' : ''}`);
      return true;
    },
    holdMove: (clientX, clientY) => {
      if (!held) return;
      const [x, y] = renderer.pageFromClient(clientX, clientY);
      const now = currentHeldRect(held);
      if (!now) return;
      // An image keeps its shape when resized, and so does a circle.
      const keepShape = held.kind === 'image' || (held.kind === 'shape' && session.ownLayer.strokes.get(held.id)?.shape === 'circle');
      const rect = heldRect(held, now, x, y, keepShape);
      if (held.kind === 'image') {
        held.index = session.placeImageByHand(held.index, rect);
      } else if (held.kind === 'hide') {
        session.placeHideByHand(held.index, rect);
      } else if (held.kind === 'shape') {
        const stroke = session.ownLayer.strokes.get(held.id);
        if (!stroke) return;
        session.setShapePoints(held.id, movedPoints(stroke, now, {
          left: Math.round(rect.left), top: Math.round(rect.top), width: Math.round(rect.width), height: Math.round(rect.height),
        }));
      } else {
        const block = session.ownLayer.texts.get(held.id);
        if (!block) return;
        block.x = Math.round(rect.left);
        block.y = Math.round(rect.top);
        if (block.height !== null) {
          block.width = Math.round(rect.width);
          Object.assign(block, layoutText(block.text, block.width, block.size, block.font));
          block.height = Math.max(Math.round(rect.height), Math.ceil(block.boxHeight));
        }
        session.setText(block);
        // The field being typed in follows its block.
        if (textEditor?.block?.id === block.id) textEditor.layout();
      }
      showHeldGrip(held);
      refreshArrangeCues();
    },
    holdEnd: () => {
      if (!held) return;
      const r = currentHeldRect(held);
      if (r) log(`${held.kind} placed at ${r.left / 100}, ${r.top / 100} cm, ${r.width / 100} by ${r.height / 100} cm`);
      if (held.kind === 'image' || held.kind === 'hide') session.sendPageNow();
      if (held.kind === 'text') session.endTextEdit(held.id, 'hold');
      if (held.kind === 'shape') session.endShapeEdit();
      if (held.kind === 'hide') session.endHideEdit();
      held = null;
      renderer.showGrip(null);
      refreshArrangeCues();
    },
    eraseAlong: (x0, y0, x1, y1) => {
      textEditor?.stop();
      session.beginErasing();
      const radius = renderer.pageLength(ERASER_RADIUS);
      const strokes = strokesTouched(session.ownLayer, x0, y0, x1, y1, radius);
      const texts = textsTouched(session.ownLayer, x0, y0, x1, y1, radius);
      session.erase(strokes);
      session.removeTexts(texts);
      return { strokes: strokes.length, texts: texts.length };
    },
    eraseEnded: (x, y, reach) => {
      // What the movement erased is one gesture to undo.
      const erased = session.endErasing();
      if (reach >= TAP_DISTANCE) {
        lastEmptyTap = 0;
        return;
      }
      // A tap of the eraser on a Hide rectangle or an image removes it,
      // for everyone (by anyone since 2026-10-18).
      let took = erased.strokes + erased.texts > 0;
      if (!took) {
        const hide = session.page.hideAt(x, y, 0);
        if (hide && session.removeHide(hide.index)) {
          log(`Hide ${hide.index + 1} removed`);
          took = true;
        } else {
          const found = session.page.imageAt(x, y, 0);
          if (found && session.removeImage(found.index)) {
            log(`Image ${found.index + 1} removed`);
            took = true;
          }
        }
      }
      if (took) {
        lastEmptyTap = 0;
        return;
      }
      // A tap on empty paper: the second within DOUBLE_TAP_MS undoes.
      const now = performance.now();
      if (now - lastEmptyTap <= DOUBLE_TAP_MS) {
        lastEmptyTap = 0;
        undo();
      } else {
        lastEmptyTap = now;
      }
    },
    previewRect: (x0, y0, x1, y1) => {
      const dragged = Math.hypot(x1 - x0, y1 - y0) >= TAP_DISTANCE;
      if (tool === 'shape') {
        // The shape as it will be, in the pen's colour and width; for a
        // line, the side the press is adding.
        if (isTapped(shape)) {
          /** @type {[number, number]} */
          const from = lineInProgress ? lineInProgress.vertices[lineInProgress.vertices.length - 1] : [x0, y0];
          if (lineInProgress || dragged) renderer.showShapePreview(straightPoints([from, [x1, y1]]), color, width, false);
        } else if (dragged) {
          renderer.showShapePreview(draggedPoints(shape, x0, y0, x1, y1, width), color, width, isFilled(shape));
        } else {
          renderer.hideShapePreview();
        }
        return;
      }
      if (dragged) {
        textEditor?.showPreview(x0, y0, x1, y1);
      } else {
        textEditor?.hidePreview();
      }
    },
    // A hovering pointer: the rubber band of a line being made, and the
    // corners of free text under it (Andy, 2026-10-14: free text is used
    // constantly, so its extent shows only then).
    hover: (clientX, clientY) => {
      const [x, y] = renderer.pageFromClient(clientX, clientY);
      if (lineInProgress && tool === 'shape') {
        const last = lineInProgress.vertices[lineInProgress.vertices.length - 1];
        renderer.showShapePreview(straightPoints([last, [x, y]]), color, width, false);
      }
      // With the Eraser, the red box says what matters, so the blue cue
      // of an implied rectangle stays away (the two had shown together).
      const implied = tool === 'eraser' ? null : impliedRectAt(x, y);
      renderer.showHoverCue(implied?.rect ?? null, implied?.kind);
      renderer.setEraseCues(tool === 'eraser' ? eraseTargetsAt(x, y) : []);
      if (tool === 'arrange') hoverHandwriting(x, y);
    },
    hoverEnded: (left = false) => {
      renderer.showHoverCue(null);
      renderer.setEraseCues([]);
      if (lineInProgress) renderer.hideShapePreview();
      // The handwriting's box stays through a press (the pointer is still
      // there) and goes when the pointer leaves the page.
      if (left && hoveredStroke) {
        hoveredStroke = null;
        refreshArrangeCues();
      }
    },
    placeRect: (x0, y0, x1, y1) => {
      if (tool === 'shape') {
        placeShape(x0, y0, x1, y1);
        return;
      }
      if (!textEditor) return;
      textEditor.hidePreview();
      const dragged = Math.hypot(x1 - x0, y1 - y0) >= TAP_DISTANCE;
      const rect = {
        left: Math.min(x0, x1), top: Math.min(y0, y1),
        width: Math.max(100, Math.round(Math.abs(x1 - x0))), height: Math.max(100, Math.round(Math.abs(y1 - y0))),
      };
      if (tool === 'hide') {
        if (dragged && session.addHide(rect)) log('Hide made');
        return;
      }
      const editing = textEditor.block;
      textEditor.stop();
      const own = textAt(session.ownLayer, x0, y0);
      if (!dragged) {
        // A tap: into our own text to change it, or free text here.  A
        // tap elsewhere while typing only finishes the block (2026-10-09).
        if (own && own !== editing) textEditor.edit(own);
        else if (!own && !editing) textEditor.begin(x0, y0, null, null, { color, size: textSize, font: textFont });
      } else {
        // A rectangle dragged open: at least one line tall.
        const line = Math.ceil(textSize * (TOP_GAP + LINE_HEIGHT));
        textEditor.begin(rect.left, rect.top, rect.width, Math.max(rect.height, line),
                         { color, size: textSize, font: textFont });
      }
    },
    onReport: (report) => {
      // To the console once, and again when the kind of device changes.
      if (report.pointerType === reportedDevice || report.pointsPerSecond === 0) return;
      reportedDevice = report.pointerType;
      log(`Input: ${report.pointerType}, pressure ${report.pressure.toFixed(2)}, ` +
          `${report.pointsPerSecond} points per second`);
    },
  };
  attachPenInput(pageElement, penTarget);

  buildToolbar({
    color, width, textSize, textFont, shape, fill,
    setTool: (value) => {
      if (value !== 'text') textEditor?.stop();
      if (value !== 'shape') finishLine();
      if (value !== tool) trace(`Tool: ${value}`);
      tool = value;
      // Only the chosen tool's settings show: the pen's and the shapes'
      // colours and line widths, the text's colours, size and font,
      // nothing for the eraser, Hide or Arrange.
      element('colors').hidden = value === 'eraser' || value === 'hide' || value === 'arrange';
      element('widths').hidden = value !== 'pen' && value !== 'shape';
      element('text_style').hidden = value !== 'text';
      if (value !== 'arrange') hoveredStroke = null;
      refreshArrangeCues();
      if (value !== 'eraser') renderer.setEraseCues([]);
    },
    setShape: (value) => {
      finishLine();
      if (value !== shape) trace(`Shape: ${value}`);
      shape = value;
      localStorage.setItem(SHAPE_KEY, value);
    },
    setFill: (value) => {
      finishLine();
      fill = value;
      localStorage.setItem(FILL_KEY, value ?? 'none');
      trace(`Fill: ${value ?? 'none'}`);
    },
    setColor: (value) => {
      color = value;
      textEditor?.restyle({ color });
    },
    setWidth: (value) => { width = value; },
    setTextSize: (value) => {
      textSize = value;
      textEditor?.restyle({ size: value });
    },
    setTextFont: (value) => {
      textFont = value;
      textEditor?.restyle({ font: value });
    },
  });

  // Zoom is each person's own; it changes nothing on the page.
  // A reloaded tab shows the page as it was last seen in it (Andy,
  // 2026-10-05: the least surprise): the zoom and the place scrolled to
  // are remembered in the tab, for the page they belong to.
  const viewKey = `kokkoja_view_${roomId}`;
  const saveView = () => {
    if (session.page.number === 0) return;
    sessionStorage.setItem(viewKey, JSON.stringify({ page: session.page.id, ...renderer.view() }));
  };
  let viewTimer = 0;
  const viewChanged = () => {
    clearTimeout(viewTimer);
    viewTimer = setTimeout(saveView, 300);
  };
  renderer.onZoomChanged = () => {
    textEditor?.layout();
    viewChanged();
  };
  // Fit and Normal scale ("A4" until 2026-10-14) are each person's own
  // (2026-10-18; the host's reached everyone from 2026-10-08 until then).
  // The page and view commands live in the Page menu (Andy, 2026-10-14:
  // seldom used, and the one row is precious).
  attachPopup(element('page_menu_button'), element('page_menu'));
  // Help (Andy, 2026-10-15): one click shows the manual, the tooltips of
  // every command gathered into one window (Ctrl-click until then); two
  // clicks within DOUBLE_TAP_MS show the note about the documentation
  // site.  Since one click opens a modal window, it must wait the
  // interval out to be told from the first of two; the shape button's
  // double click needs no such wait, its single click having chosen the
  // tool already.  Two clicks are counted here rather than by the
  // browser's dblclick, which a pen or a touch screen does not always
  // send.
  const manualDialog = /** @type {HTMLDialogElement} */ (element('manual_dialog'));
  const helpNote = attachPopup(element('help'), element('help_popup'), undefined, false);
  const showManual = () => {
    helpNote.close();
    const body = element('manual_text');
    body.replaceChildren(manualList(document));
    manualDialog.showModal();
    body.focus();
  };
  /** @type {ReturnType<typeof setTimeout> | null} */
  let helpClick = null;
  element('help').addEventListener('click', () => {
    if (helpClick !== null) {
      clearTimeout(helpClick);
      helpClick = null;
      helpNote.open();
    } else {
      helpClick = setTimeout(() => {
        helpClick = null;
        showManual();
      }, DOUBLE_TAP_MS);
    }
  });
  element('manual_close').addEventListener('click', () => manualDialog.close());
  element('copy_manual').addEventListener('click', async () => {
    try {
      await navigator.clipboard.writeText(manualText(document));
      element('copy_manual').textContent = 'Copied';
      setTimeout(() => { element('copy_manual').textContent = 'Copy'; }, 1500);
    } catch (error) {
      log(`The manual could not be copied: ${error}`);
    }
  });
  element('zoom_fit').addEventListener('click', () => renderer.fit());
  element('zoom_sheet').addEventListener('click', () => renderer.sheetScale());
  // "Expand" makes the sheet itself fill the window at the present zoom,
  // in both directions; nothing on it moves (Andy, 2026-10-08 for the
  // width, both ways 2026-10-11).
  element('zoom_expand').addEventListener('click', () => {
    const room = renderer.available();
    const expanded = session.expandPage(room.width / renderer.cssPixelsPerUnit,
                                         room.height / renderer.cssPixelsPerUnit);
    if (expanded) {
      log(`Page expanded to ${session.page.baseWidth / 100} by ${session.page.baseHeight / 100} cm`);
    }
  });
  element('zoom_shrink').addEventListener('click', () => {
    if (session.shrinkPage()) {
      log(`Page shrunk to ${session.page.baseWidth / 100} by ${session.page.baseHeight / 100} cm`);
    }
  });
  try {
    const seen = JSON.parse(sessionStorage.getItem(viewKey) ?? 'null');
    if (seen && seen.page === session.page.id && (seen.zoom === 'fit' || seen.zoom > 0) &&
        Number.isFinite(seen.x) && Number.isFinite(seen.y)) {
      renderer.showView(seen);
    }
  } catch {
    // Nothing usable was remembered: the page is shown as a new one is.
  }
  element('desk').addEventListener('scroll', viewChanged, { passive: true });
  window.addEventListener('pagehide', saveView);

  // Everyone has every button (Andy and his teacher, 2026-10-18; until
  // then New, Image, Hide, Expand and Shrink were the host's and the
  // co-hosts').
  element('new_page').addEventListener('click', () => {
    log('New page asked for');
    session.newPage();
  });

  /**
   * Add an image file to the page, below the images already on it, at
   * its natural size; moving and scaling it is done by hand afterwards.
   * @param {Blob} file
   */
  async function loadImage(file) {
    if (session.page.number === 0) return;
    try {
      if (!session.addImage(await prepareImage(file))) {
        throw new Error('The page has no room for another image.');
      }
    } catch (error) {
      passingNotice(error instanceof Error && error.message ? error.message : 'That file is not an image.', true);
    }
  }
  const chooser = new NotebookChooser((chosenNotebook, pageName, text) => {
    const kept = pageFromSvg(text);
    if (!kept) {
      passingNotice(`${pageName} is not a Kokkoja page.`, true);
      return;
    }
    log(`Notebook page loaded: ${chosenNotebook} / ${pageName}`);
    session.loadPage(kept.page);
  }, () => {
    // A deleted notebook leaves the toolbar's list.
    showNotebooks(sessionPicker).catch(() => {});
  });
  element('load_notebook_page').addEventListener('click', () => { chooser.open(writer.notebook); });

  // Image files are chosen, and pictures of the page exported, through a
  // chooser of our own, which begins where the notebooks are.
  const fileChooser = new FileChooser();
  element('load_image').addEventListener('click', async () => {
    if (notebooksInBrowser()) {
      // No local program to look through the files: the browser's own chooser.
      const file = await pickImageFile();
      if (file) loadImage(file);
      return;
    }
    const file = await fileChooser.chooseImage();
    if (file) loadImage(file);
  });

  /**
   * Save a picture of the page as it is shown here (layers hidden on this
   * screen are left out) where the user chooses.
   */
  async function exportPage() {
    const page = session.page;
    if (page.number === 0) return;
    textEditor?.stop();
    // Anything still waiting to be saved is saved first, so that its
    // "Saved" does not take the place of "Exported" a moment later.
    await writer.writeNow();
    const now = new Date();
    const day = [now.getFullYear(), now.getMonth() + 1, now.getDate()]
      .map((part) => String(part).padStart(2, '0')).join('_');
    if (notebooksInBrowser()) {
      download(`${day}_page_${page.number}.png`,
               await pageToPng(page, { session: roomId, hostId: session.hostId, hidden: renderer.hidden }));
      log(`Exported page ${page.number} to the downloads`);
      passingNotice('Exported to your downloads');
      return;
    }
    const saved = await fileChooser.chooseWhereToSave(
      'Export', `${day}_page_${page.number}.png`,
      async (directory, name, replace) => writePicture(
        directory, name,
        await pageToPng(page, { session: roomId, hostId: session.hostId, hidden: renderer.hidden }),
        replace));
    if (saved === null) return;
    log(`Exported page ${page.number} to ${saved}`);
    passingNotice(`Exported to ${saved}`);
  }
  element('export_page').addEventListener('click', () => { exportPage(); });

  // The notebook: choosing one, making one, and keeping the page.
  const sessionPicker = new NotebookPicker(
    select('session_notebook'), input('session_notebook_name'),
    (message) => { if (message) passingNotice(message, true); },
    (chosen) => {
      if (chosen === writer.notebook) return;
      sessionStorage.setItem(notebookKey, chosen);
      localStorage.setItem(NOTEBOOK_KEY, chosen);
      writer.changeNotebook(chosen, (text) => {
        const saved = pageFromSvg(text);
        return saved !== null && saved.session === roomId && saved.page.id === session.page.id;
      }).then(() => showNotebooks(sessionPicker)).catch(() => {});
    });
  sessionPicker.fill([], notebook);
  sessionPicker.current = notebook;
  showNotebooks(sessionPicker).catch(() => {});
  element('keep_page').addEventListener('click', async () => {
    const { kept, new: made } = await writer.keepPage();
    log(kept ? `${made ? 'Kept' : 'Already kept'} as ${kept}` : 'Nothing to keep');
    // In the browser the notebook is out of reach of the user's own
    // programs: a page kept just now is also downloaded, which is how a
    // page gets out.  One already kept was downloaded then, and is not
    // again (Andy, 2026-10-06: a second press should do nothing new).
    if (kept && made && notebooksInBrowser()) {
      const text = await readPage(writer.notebook, kept);
      if (text !== null) download(kept, new Blob([text], { type: 'image/svg+xml' }));
    }
  });

  const sessionDialog = /** @type {HTMLDialogElement} */ (element('session_dialog'));
  element('session_button').addEventListener('click', () => {
    showLog();
    sessionDialog.showModal();
  });
  element('session_close').addEventListener('click', () => sessionDialog.close());

  // The log in the Session window, kept current while the window is open.
  const logView = element('event_log');
  function showLog() {
    logView.textContent = logText();
    logView.scrollTop = logView.scrollHeight;
  }
  watchLog(() => { if (sessionDialog.open) showLog(); });
  element('copy_log').addEventListener('click', async () => {
    try {
      await navigator.clipboard.writeText(logText());
      element('copy_log').textContent = 'Copied';
      setTimeout(() => { element('copy_log').textContent = 'Copy'; }, 1500);
    } catch {
      // The text is selectable; the user can copy it by hand.
    }
  });

  element('copy_invitation').addEventListener('click', async () => {
    const field = input('invitation');
    field.select();
    try {
      await navigator.clipboard.writeText(field.value);
      element('copy_invitation').textContent = 'Copied';
      setTimeout(() => { element('copy_invitation').textContent = 'Copy'; }, 1500);
    } catch {
      // The field is selected; the user can copy it with the keyboard.
    }
  });

  if (notebooksInBrowser()) {
    element('keep_page').setAttribute('data-help',
      'Keep a dated copy of the page as it is now in this browser’s notebook, and download it. ' +
      'The page is also kept by itself whenever it is about to be replaced.');
    element('export_page').setAttribute('data-help',
      'Download a picture of the page, as it is shown here, as a PNG file.');
  }

  session.start();
  // For looking at the session from the browser's console during development.
  Object.assign(window, { kokkoja: {
    session, renderer, writer, loadImage, textEditor, logText,
    manualText: () => manualText(document),
    notebook: { listNotebooks, listKeptPages, readPage, readCurrentPage, notebooksInBrowser, discardNotebook, discardPage },
  } });
}

/**
 * Mark one button of a group as chosen.
 * @param {Element} group
 * @param {Element} chosen
 */
function press(group, chosen) {
  for (const button of group.querySelectorAll('button')) {
    button.setAttribute('aria-pressed', String(button === chosen));
  }
}

/** The closers of every popup, so that opening one closes the others. @type {Set<() => void>} */
const popupClosers = new Set();

/**
 * A popup of Kokkoja's own below a toolbar button (the colour palette,
 * the Page menu, the Help note, the shapes; no system menu or dialog,
 * 2026-10-14).  The button's press opens it below the button, left edges
 * aligned, or right edges when it would run past the window; a press
 * anywhere else, Escape, or a press on an item marked `.menu_item`
 * closes it.
 * @param {HTMLElement} button
 * @param {HTMLElement} popup
 * @param {() => void} [onOpen]  Fill the popup before it shows.
 * @param {boolean} [byClick]  Whether the button's click opens and closes
 *     it; the Shape button opens its menu by a hold instead.
 * @returns {{open: () => void, close: () => void}}
 */
export function attachPopup(button, popup, onOpen, byClick = true) {
  const close = () => {
    popup.hidden = true;
    button.setAttribute('aria-expanded', 'false');
  };
  popupClosers.add(close);
  const open = () => {
    // One popup at a time (a keyboard's Enter opens without the press
    // that would close the other).
    for (const other of popupClosers) other();
    onOpen?.();
    const box = button.getBoundingClientRect();
    // Measured at the window's left edge: placed near the right edge, an
    // absolute box shrinks to the room left and its text wraps.
    popup.style.left = '0px';
    popup.style.top = `${box.bottom + 6}px`;
    popup.hidden = false;
    button.setAttribute('aria-expanded', 'true');
    const width = popup.getBoundingClientRect().width;
    const fits = box.left + width <= window.innerWidth - 8;
    popup.style.left = `${fits ? box.left : Math.max(8, box.right - width)}px`;
  };
  if (byClick) {
    button.addEventListener('click', () => {
      if (popup.hidden) open();
      else close();
    });
  }
  popup.addEventListener('click', (event) => {
    const item = /** @type {Element} */ (event.target).closest('.menu_item');
    // A label's click reaches here twice, once as the input's; close on the second.
    if (item && !(event.target instanceof HTMLLabelElement)) close();
  });
  document.addEventListener('pointerdown', (event) => {
    const target = /** @type {Node} */ (event.target);
    if (!popup.hidden && !popup.contains(target) && !button.contains(target)) close();
  });
  document.addEventListener('keydown', (event) => {
    if (event.key === 'Escape' && !popup.hidden) close();
  });
  return { open, close };
}

/**
 * @param {object} options
 * @param {string} options.color  Initial color.
 * @param {number} options.width  Initial width.
 * @param {number} options.textSize
 * @param {import('./page_model.js').TextFont} options.textFont
 * @param {Shape} options.shape  The Shape tool's shape to begin with.
 * @param {string | null} options.fill  Its fill colour, or none.
 * @param {(tool: Tool) => void} options.setTool
 * @param {(shape: Shape) => void} options.setShape
 * @param {(fill: string | null) => void} options.setFill
 * @param {(color: string) => void} options.setColor
 * @param {(width: number) => void} options.setWidth
 * @param {(size: number) => void} options.setTextSize
 * @param {(font: import('./page_model.js').TextFont) => void} options.setTextFont
 */
function buildToolbar(options) {
  const shapeButton = element('shape_tool');
  /** The Shape button's hold has just opened the menu: the click that follows chooses nothing. */
  let shapeMenuOpened = false;
  /** @param {Element} button */
  const chooseTool = (button) => {
    // The tool buttons stand in two groups since 2026-10-11 (Pen, Text
    // and the shapes before the colours they use, Hide, Arrange and
    // Eraser after), so the pressed one is marked among all of them, not
    // among siblings.
    for (const other of document.querySelectorAll('[data-tool]')) {
      other.setAttribute('aria-pressed', String(other === button));
    }
    options.setTool(/** @type {Tool} */ (button.getAttribute('data-tool')));
  };
  for (const button of document.querySelectorAll('[data-tool]')) {
    button.addEventListener('click', () => {
      if (button === shapeButton && shapeMenuOpened) {
        shapeMenuOpened = false;
        return;
      }
      chooseTool(button);
    });
  }

  // The Shape button names the current shape, Rectangle to begin with,
  // and a quick press chooses the tool; a press held for HOLD_MS, a
  // press with the right button (a pen's barrel button) or a double
  // click (Andy, 2026-10-14) opens the menu of shapes, and the shape
  // chosen there is the button's from then on and the tool is chosen
  // (Andy, 2026-10-14: one button for any number of shapes).  The menu
  // marks the current shape, and ends with the fill colours.  The button
  // keeps the width of its widest name, so that nothing in the toolbar
  // moves when the shape changes.
  const shapeMenu = element('shape_menu');
  const shapePopup = attachPopup(shapeButton, shapeMenu, undefined, false);
  let widest = 0;
  for (const name of Object.values(SHAPE_NAMES)) {
    shapeButton.textContent = name;
    widest = Math.max(widest, shapeButton.getBoundingClientRect().width);
  }
  shapeButton.style.minWidth = `${Math.ceil(widest)}px`;
  /** @param {Shape} shape */
  const showShape = (shape) => {
    shapeButton.textContent = SHAPE_NAMES[shape];
    for (const item of shapeMenu.querySelectorAll('[data-shape]')) {
      item.setAttribute('aria-checked', String(item.getAttribute('data-shape') === shape));
    }
  };
  showShape(options.shape);
  /** @type {ReturnType<typeof setTimeout> | null} */
  let shapeHold = null;
  const cancelShapeHold = () => {
    if (shapeHold !== null) clearTimeout(shapeHold);
    shapeHold = null;
  };
  const openShapeMenu = () => {
    shapeMenuOpened = true;
    shapePopup.open();
  };
  shapeButton.addEventListener('pointerdown', (event) => {
    if (event.button !== 0) return;
    shapeMenuOpened = false;
    cancelShapeHold();
    shapeHold = setTimeout(() => {
      shapeHold = null;
      openShapeMenu();
    }, HOLD_MS);
  });
  for (const type of ['pointerup', 'pointercancel', 'pointerleave']) shapeButton.addEventListener(type, cancelShapeHold);
  shapeButton.addEventListener('contextmenu', (event) => {
    event.preventDefault();
    cancelShapeHold();
    openShapeMenu();
    shapeMenuOpened = false;     // No click follows a secondary press.
  });
  // A double click, told from two clicks within DOUBLE_TAP_MS rather than
  // from the browser's dblclick event, which a pen or a touch screen does
  // not always send; both clicks have chosen the tool already.
  let lastShapeClick = -Infinity;
  shapeButton.addEventListener('click', () => {
    const now = performance.now();
    if (now - lastShapeClick <= DOUBLE_TAP_MS) {
      lastShapeClick = -Infinity;
      openShapeMenu();
      shapeMenuOpened = false;
    } else {
      lastShapeClick = now;
    }
  });
  // The fill colours: the toolbar's colours, those added to it, and
  // "none", a state of their own (Andy, 2026-10-14).  A press keeps the
  // menu open.
  const fills = element('shape_fill');
  /** @param {string | null} value */
  const addFill = (value) => {
    const button = document.createElement('button');
    button.type = 'button';
    button.className = `swatch${value === null ? ' none' : ''}`;
    if (value !== null) button.style.setProperty('--swatch', value);
    button.setAttribute('aria-label', value === null ? 'No fill' : `Fill ${value}`);
    button.setAttribute('data-help', value === null ? 'Draw the shapes unfilled.' : 'Fill the shapes with this color.');
    button.setAttribute('aria-pressed', String(value === options.fill));
    button.addEventListener('click', () => {
      press(fills, button);
      options.setFill(value);
    });
    fills.append(button);
  };
  addFill(null);
  for (const value of COLORS) addFill(value);
  for (const item of shapeMenu.querySelectorAll('[data-shape]')) {
    item.addEventListener('click', () => {
      const chosen = shapeNamed(item.getAttribute('data-shape'));
      options.setShape(chosen);
      showShape(chosen);
      chooseTool(shapeButton);
    });
  }

  const colors = element('colors');
  const palette = element('color_palette');
  const more = element('more_colors');

  /** @param {string} color */
  const addSwatch = (color) => {
    const button = document.createElement('button');
    button.type = 'button';
    button.className = 'swatch';
    button.style.setProperty('--swatch', color);
    button.setAttribute('aria-label', `Color ${color}`);
    button.setAttribute('data-help', 'Write in this color.');
    button.setAttribute('aria-pressed', String(color === options.color));
    button.addEventListener('click', () => {
      press(colors, button);
      options.setColor(color);
    });
    colors.insertBefore(button, more);
    return button;
  };
  /** @type {string[]} */
  let added = [];
  try {
    added = JSON.parse(localStorage.getItem(COLORS_KEY) ?? '[]');
  } catch {
    added = [];
  }
  for (const color of COLORS) addSwatch(color);
  for (const color of added) {
    if (/^#[0-9a-f]{6}$/.test(color)) {
      addSwatch(color);
      addFill(color);
    }
  }

  // "+" opens a palette below it; a chosen colour becomes a swatch before
  // the "+" and is the colour from then on.
  /** @param {string} color */
  const choose = (color) => {
    palette.hidden = true;
    const existing = [...colors.querySelectorAll('.swatch')]
      .find((swatch) => swatch.getAttribute('aria-label') === `Color ${color}`);
    const button = existing ?? addSwatch(color);
    if (!existing) {
      added.push(color);
      localStorage.setItem(COLORS_KEY, JSON.stringify(added));
      addFill(color);     // The shapes' fills follow the toolbar's colours.
    }
    press(colors, button);
    options.setColor(color);
  };
  // A colour that is already in the row is missing from the palette, its
  // place in the grid left empty (Andy's teacher, 2026-10-08).
  const inRow = () => new Set([...colors.querySelectorAll('.swatch:not(.more)')]
    .map((swatch) => swatch.getAttribute('aria-label')?.replace('Color ', '')));
  const showPalette = () => {
    palette.replaceChildren();
    const taken = inRow();
    for (const color of PALETTE) {
      if (taken.has(color)) {
        const gap = document.createElement('span');
        gap.className = 'swatch missing';
        palette.append(gap);
        continue;
      }
      const button = document.createElement('button');
      button.type = 'button';
      button.className = 'swatch';
      button.style.setProperty('--swatch', color);
      button.setAttribute('aria-label', `Color ${color}`);
      button.addEventListener('click', () => choose(color));
      palette.append(button);
    }
  };
  attachPopup(more, palette, showPalette);

  const widths = element('widths');
  for (const width of WIDTHS) {
    const button = document.createElement('button');
    button.type = 'button';
    button.className = 'width_choice';
    button.setAttribute('aria-label', `Line width ${width / 10} mm`);
    button.setAttribute('data-help', `Write with a ${width / 10} mm line. Pressing harder widens it.`);
    button.setAttribute('aria-pressed', String(width === options.width));
    const line = document.createElement('span');
    line.style.height = `${Math.max(1, Math.round(width / 2.5))}px`;
    button.append(line);
    button.addEventListener('click', () => {
      press(widths, button);
      options.setWidth(width);
    });
    widths.append(button);
  }

  // The text size is a number of points; any size works, since the fonts
  // scale.  The font is one of the three generic families.
  const size = input('text_size');
  size.value = String(Math.round(options.textSize / UNITS_PER_POINT));
  const applySize = () => {
    const points = Math.max(LEAST_TEXT_POINTS, Math.min(MOST_TEXT_POINTS, Number(size.value) || 0));
    size.value = String(points);
    options.setTextSize(Math.round(points * UNITS_PER_POINT * 10) / 10);
  };
  size.addEventListener('change', applySize);
  size.addEventListener('input', () => {
    if (Number(size.value) >= LEAST_TEXT_POINTS) applySize();
  });

  const font = /** @type {HTMLSelectElement} */ (document.getElementById('text_font'));
  font.value = options.textFont;
  font.style.fontFamily = options.textFont;
  font.addEventListener('change', () => {
    const chosen = TEXT_FONTS.find((family) => family === font.value) ?? 'sans-serif';
    font.style.fontFamily = chosen;
    options.setTextFont(chosen);
  });
}

/**
 * @param {Session} session
 * @param {Renderer} renderer
 */
function showParticipants(session, renderer) {
  const list = element('participants');
  list.replaceChildren();
  for (const participant of session.participantList()) {
    const item = document.createElement('li');
    const present = participant.self || participant.peer !== null;
    item.classList.toggle('absent', !present);

    const label = document.createElement('label');
    const visible = document.createElement('input');
    visible.type = 'checkbox';
    visible.checked = !renderer.hidden.has(participant.id);
    visible.setAttribute('aria-label', 'Shown');
    visible.setAttribute('data-help', 'Show or hide this participant’s writing on your screen only.');
    visible.addEventListener('change', () => renderer.setLayerVisible(participant.id, visible.checked));
    label.append(visible, ` ${participant.name}`);
    item.append(label);

    const roles = [];
    if (participant.self) roles.push('you');
    if (!present) roles.push('away');
    if (roles.length) {
      const role = document.createElement('span');
      role.className = 'role';
      role.textContent = roles.join(', ');
      item.append(role);
    }
    list.append(item);
  }
}

// Startup.

/** @param {any} error */
function failedToEnter(error) {
  entered = false;
  showStart(String(error?.message ?? error));
}

element('start_session').addEventListener('click', () => {
  startSession().catch(failedToEnter);
});
element('join_session').addEventListener('click', () => {
  joinSession().catch(failedToEnter);
});

/**
 * Begin where this browser tab left off.  A tab that was in a session and
 * is reloaded goes straight back into it.  A tab that arrives with an
 * invitation in its address shows the start screen with the invitation
 * filled in, so that the name and the notebook can be chosen.
 */
async function resume() {
  const invitationText = invitationInAddress();
  const invitation = parseInvitation(invitationText);
  const name = localStorage.getItem(NAME_KEY);
  if (invitation && name) {
    const { roomId } = await deriveSession(invitation.secret);
    const notebook = sessionStorage.getItem(notebookKeyFor(roomId));
    if (notebook) {
      await enterSession(invitationText, name, notebook);
      return;
    }
  }
  showStart();
}

enableTooltips();
guardSelection();
log(`Browser: ${navigator.userAgent}; window ${innerWidth}×${innerHeight}, pixel ratio ${devicePixelRatio}`);

const token = addressValues().get('token');
if (token) {
  rememberToken(token);
  setAddress(invitationInAddress());
}

/** Learn how this client was served, then begin. */
async function startUp() {
  const served = await (await fetch('config.json')).json();
  config = {
    relay: String(served.relay ?? ''),
    storage: served.storage === 'browser' ? 'browser' : 'files',
    site: String(served.site ?? ''),
  };
  if (config.storage === 'browser') useBrowserStorage();
  await resume();
}

if (!window.isSecureContext) {
  showStart('Kokkoja must be opened from this computer (http://localhost) or over HTTPS.');
} else {
  startUp().catch(failedToEnter);
}