kokkoja-rel/client/js/session.js

// @ts-check
/**
 * A session: the page, the participants, and the messages that keep every
 * participant's copy of the page the same.
 *
 * The rule that makes this simple is that each layer has one writer.  A
 * participant is the authority on their own layer, so there are no
 * conflicting edits to reconcile.  See notes/protocol.md for the messages.
 */

import {
  decrypt, encrypt, fromBase64, hashOf, randomId, signClaim, toBase64, verifyClaim,
} from './crypto.js';
import {
  MAX_STROKE_POINTS, POINT_SIZE, Page, appendPoints, cleanName, cleanPageDescriptor,
  cleanStroke, cleanText, placeImage, pointCount, sheetHolding, validId, validPoints,
  MAX_PAGE_HIDES,
} from './page_model.js';
import { RelayConnection } from './relay_connection.js';

/** @typedef {import('./crypto.js').Identity} Identity */
/** @typedef {import('./page_model.js').Layer} Layer */
/** @typedef {import('./page_model.js').Stroke} Stroke */
/** @typedef {import('./page_model.js').PageImage} PageImage */
/** @typedef {import('./page_model.js').TextBlock} TextBlock */
/** @typedef {import('./page_model.js').HideRect} HideRect */

/**
 * One gesture of ours, with what is needed to take it back: a stroke
 * drawn; strokes and text blocks erased by one movement of the eraser;
 * a text block edited (made, typed into, moved or resized), with the
 * block as it was before, or null if it was new; a Hide rectangle made,
 * moved or removed, likewise.  Images are not here: Andy, 2026-10-11,
 * the undo set is what is cheap, and an image's data is not kept.
 * @typedef {{kind: 'stroke', id: string}
 *   | {kind: 'erase', strokes: Stroke[], texts: TextBlock[]}
 *   | {kind: 'text', id: string, before: TextBlock | null}
 *   | {kind: 'hide', id: string, before: HideRect | null}} UndoEntry
 */

/**
 * @typedef {object} Participant
 * @property {string} id
 * @property {string} name
 * @property {number | null} peer  Relay peer number; null when not connected.
 * @property {boolean} self
 * @property {boolean} host
 */

/**
 * @typedef {object} SessionEvents
 * @property {(layerId: string) => void} onLayerReplaced
 *     A layer's contents changed as a whole and must be redrawn.
 * @property {(layerId: string, stroke: Stroke) => void} onStrokeProgress
 *     A stroke gained points or was finished.
 * @property {(layerId: string, block: TextBlock) => void} onTextChanged
 *     A text block was made or changed.
 * @property {(oldPage: Page, newPage: Page) => void} onPageReplaced
 *     The session moved to another page.
 * @property {() => void} onImagesChanged
 *     An image was added to the page, or the bytes of one have arrived.
 * @property {(byImages: boolean) => void} onPageResized
 *     The page grew (or shrank) around its contents; `byImages` if an
 *     image was added or taken back, not if writing reached an edge.
 * @property {() => void} onPermissionsChanged
 *     Who may start pages changed.
 * @property {(command: 'fit' | 'sheet') => void} onViewCommand
 *     The host pressed "Fit" or "A4": everyone's view follows (Andy,
 *     2026-10-08), each in their own window.
 * @property {() => void} onParticipantsChanged
 * @property {(status: 'connecting' | 'connected' | 'failed', detail?: string) => void} onStatus
 * @property {() => void} [onUndoChanged]
 *     Whether there is something of ours to undo may have changed.
 */

/**
 * @typedef {object} SessionOptions
 * @property {string} relay
 * @property {string} roomId
 * @property {CryptoKey} key
 * @property {Identity} identity
 * @property {string} hostId
 * @property {string} name     Our display name.
 * @property {Page} [page]     The page as last saved, when returning to
 *     the session.
 */

/** Approximate size at which a layer snapshot is split into messages. */
const SNAPSHOT_CHUNK_CHARACTERS = 100000;

/** How many of one's own gestures can be undone, the latest first. */
export const UNDO_LIMIT = 100;

/** An image is sent in pieces of this many bytes. */
const IMAGE_CHUNK_BYTES = 96 * 1024;

/** While an image is dragged by hand, its new place is sent this often. */
const PAGE_SEND_INTERVAL_MS = 40;

/**
 * The points of one's own stroke are sent at most this often: the first
 * at once, the rest gathered (Andy, 2026-10-10, after the load test:
 * one message per pointer event was 60 to 120 messages a second from
 * each writer, and the relay's limit is the message rate).  The delay
 * others see is at most this, under what Andy felt as no lag.  The
 * writer's own screen is not delayed.
 */
export const POINT_SEND_INTERVAL_MS = 25;

export class Session {
  /**
   * @param {SessionOptions} options
   * @param {SessionEvents} events
   */
  constructor(options, events) {
    this.roomId = options.roomId;
    this.key = options.key;
    this.identity = options.identity;
    this.hostId = options.hostId;
    this.name = cleanName(options.name) || 'Unnamed';
    /** @type {ReturnType<typeof setTimeout> | null}  A page descriptor waiting to be sent. */
    this.pageSendTimer = null;
    /** @type {{id: string, at: number, points: number[]} | null}  Points of one's own stroke not yet sent. */
    this.pendingPoints = null;
    /** @type {ReturnType<typeof setTimeout> | null} */
    this.pointSendTimer = null;
    this.lastPointSend = 0;
    /**
     * Our gestures on this page, the latest last (Andy, 2026-10-11: the
     * latest gesture is undone, then the one before).  Cleared with the
     * page; a reloaded tab starts with none.
     * @type {UndoEntry[]}
     */
    this.undoStack = [];
    /** @type {{strokes: Stroke[], texts: TextBlock[]} | null}  The eraser's movement in progress. */
    this.erasing = null;
    /** @type {{id: string, owner: string, before: TextBlock | null} | null}  A text block being edited. */
    this.textEditing = null;
    /** @type {{id: string, before: HideRect} | null}  A Hide rectangle being moved. */
    this.hideEditing = null;
    this.events = events;

    this.isHost = this.identity.id === this.hostId;
    /** @type {Set<string>}  Guests the host allows to start pages. */
    this.permitted = new Set();
    // The host begins a session with page 1.  A guest shows a placeholder
    // until the host's page arrives.
    this.page = options.page ??
      (this.isHost ? new Page(undefined, undefined, randomId(), 1) : new Page());
    this.ownLayer = this.page.layer(this.identity.id, this.name);
    this.page.recomputeStrokeBounds();
    this.page.updateExtent();

    /** @type {Map<string, Participant>} */
    this.participants = new Map();
    this.participants.set(this.identity.id, {
      id: this.identity.id, name: this.name, peer: null, self: true,
      host: this.identity.id === this.hostId,
    });
    for (const layer of this.page.layers.values()) {
      if (layer.id === this.identity.id) continue;
      this.participants.set(layer.id, {
        id: layer.id, name: layer.name || 'Unnamed', peer: null, self: false,
        host: layer.id === this.hostId,
      });
    }

    /** @type {Map<number, string>}  Relay peer number to participant id. */
    this.peerIds = new Map();
    /** @type {Set<number>}  Peers the relay says are present. */
    this.presentPeers = new Set();
    /** @type {Map<number, number>}  Last sequence number seen from a peer. */
    this.lastSeq = new Map();
    /** @type {Map<string, {name: string, strokes: Stroke[], texts: TextBlock[]}>}  Snapshots being received. */
    this.incoming = new Map();
    /**
     * The bytes of the page's images received so far, by each image's hash.
     * @type {Map<string, {buffer: Uint8Array<ArrayBuffer>, received: number}>}
     */
    this.imageTransfers = new Map();
    /** @type {Map<string, Set<number>>}  Peers waiting for an image from us, by its hash. */
    this.imageRequests = new Map();
    /**
     * The peer whose announcement moved us to the present page.  Its
     * snapshots of any layer of that page are accepted: when a page is
     * loaded from a notebook, that peer sends the writing kept with it.
     */
    this.pageSourcePeer = 0;

    this.myPeer = 0;
    /** The peer asked for the layers of absent participants. */
    this.statePeer = 0;
    this.stateComplete = true;
    /**
     * True while our own layer may be completed from another participant's
     * copy: on the first connection after the browser page was loaded, when
     * the others may hold strokes of ours that we had not yet saved.
     */
    this.recoverOwnLayer = false;
    this.firstConnection = true;

    /** Counts connections, so that work queued for an old one is dropped. */
    this.epoch = 0;
    this.seq = 0;
    this.sendChain = Promise.resolve();
    this.receiveChain = Promise.resolve();

    /** @type {Stroke | null}  The stroke we are drawing. */
    this.currentStroke = null;

    this.connection = new RelayConnection(options.relay, options.roomId, {
      onWelcome: (peer, peers) => this.welcomed(peer, peers),
      onJoin: (peer) => { this.presentPeers.add(peer); },
      onLeave: (peer) => this.peerLeft(peer),
      onMessage: (peer, body) => this.received(peer, body),
      onStatus: (status, detail) => {
        if (status !== 'connected') this.disconnected();
        this.events.onStatus(status, detail);
      },
    });
  }

  start() {
    this.connection.connect();
  }

  /** @returns {Participant[]}  Ourselves first, then the others by name. */
  participantList() {
    return [...this.participants.values()].sort((a, b) =>
      Number(b.self) - Number(a.self) || a.name.localeCompare(b.name) || a.id.localeCompare(b.id));
  }

  /** @returns {string[]}  Layer ids from bottom to top. */
  layerOrder() {
    return this.page.layerOrder(this.hostId);
  }

  // Pages.

  /** @returns {boolean}  Whether we may start pages: the host, or a guest the host allows. */
  mayMakePages() {
    return this.isHost || this.permitted.has(this.identity.id);
  }

  /**
   * The host's "Fit" or "A4" goes to everyone.  Only the host's does.
   * @param {'fit' | 'sheet'} command
   */
  sendViewCommand(command) {
    if (!this.isHost) return;
    this.send(0, { type: 'view', command });
  }

  /**
   * Allow a guest to start pages, or stop allowing it.  Only the host may.
   * @param {string} id
   * @param {boolean} allowed
   */
  setPermitted(id, allowed) {
    if (!this.isHost || id === this.identity.id) return;
    if (allowed) this.permitted.add(id);
    else this.permitted.delete(id);
    this.send(0, { type: 'permissions', ids: [...this.permitted] });
    this.events.onPermissionsChanged();
  }

  /**
   * The page's rectangle may have changed with its contents.
   * @param {boolean} [byImages]  Whether its images were what changed.
   */
  extentChanged(byImages = false) {
    if (this.page.updateExtent()) this.events.onPageResized(byImages);
  }

  /**
   * Start a new blank page for everyone.  The host may; so may a guest the
   * host allows.
   */
  newPage() {
    if (!this.mayMakePages()) return;
    const page = new Page(undefined, undefined, randomId(), this.page.number + 1);
    page.updateExtent();
    this.replacePage(page);
    this.send(0, { type: 'page', page: page.descriptor() });
  }

  /**
   * Add an image to the page for everyone, below the images already on it.
   * The host may; so may a guest the host allows.
   * @param {object} prepared
   * @param {PageImage['type']} prepared.type
   * @param {Uint8Array<ArrayBuffer>} prepared.data
   * @param {string} prepared.hash
   * @param {number} prepared.naturalWidth   In page units.
   * @param {number} prepared.naturalHeight
   * @returns {boolean}  False if the page has no room for it.
   */
  addImage(prepared) {
    const page = this.page;
    if (!this.mayMakePages() || page.number === 0) return false;
    const place = placeImage(page, prepared.naturalWidth, prepared.naturalHeight);
    if (!page.hasRoomForImage(prepared.data.length, place.y + place.height)) return false;
    /** @type {PageImage} */
    const image = {
      type: prepared.type, size: prepared.data.length, hash: prepared.hash, data: prepared.data,
      x: place.x, y: place.y, width: place.width, height: place.height, by: this.identity.id,
    };
    page.images.push(image);
    page.imagesRevision += 1;
    page.baseLeft = place.sheet.left;
    page.baseTop = place.sheet.top;
    page.baseWidth = place.sheet.width;
    page.baseHeight = place.sheet.height;
    this.send(0, { type: 'page', page: page.descriptor() });
    this.sendImage(0, image);
    this.events.onImagesChanged();
    this.extentChanged(true);
    return true;
  }

  // Hide rectangles: page objects like images, for those who may make pages.

  /**
   * Hide part of the page for everyone.
   * @param {import('./page_model.js').Rectangle} rect
   * @returns {boolean}  Whether a Hide rectangle was made.
   */
  addHide(rect) {
    const page = this.page;
    if (!this.mayMakePages() || page.number === 0 || page.hides.length >= MAX_PAGE_HIDES) return false;
    const hide = {
      id: randomId(), x: Math.round(rect.left), y: Math.round(rect.top),
      width: Math.round(rect.width), height: Math.round(rect.height),
    };
    page.hides.push(hide);
    this.sheetAround(rect);
    page.imagesRevision += 1;
    this.sendPageNow();
    this.events.onImagesChanged();
    this.extentChanged(true);
    this.pushUndo({ kind: 'hide', id: hide.id, before: null });
    return true;
  }

  /**
   * A Hide rectangle is about to be moved or resized by hand: remember
   * it as it is, so that the whole movement can be undone as one.
   * @param {string} id
   */
  beginHideEdit(id) {
    const hide = this.page.hides.find((one) => one.id === id);
    if (hide) this.hideEditing = { id, before: { ...hide } };
  }

  /** The movement ended: record it if the rectangle changed. */
  endHideEdit() {
    const editing = this.hideEditing;
    this.hideEditing = null;
    if (!editing) return;
    const hide = this.page.hides.find((one) => one.id === editing.id);
    if (hide && !sameRect(hide, editing.before)) {
      this.pushUndo({ kind: 'hide', id: editing.id, before: editing.before });
    }
  }

  /**
   * Give a Hide rectangle a new rectangle: moved or resized by hand.
   * @param {number} index
   * @param {import('./page_model.js').Rectangle} rect
   */
  placeHideByHand(index, rect) {
    const page = this.page;
    const hide = page.hides[index];
    if (!this.mayMakePages() || !hide) return;
    hide.x = Math.round(rect.left);
    hide.y = Math.round(rect.top);
    hide.width = Math.round(rect.width);
    hide.height = Math.round(rect.height);
    this.sheetAround(rect);
    this.imagesChangedByHand();
  }

  /**
   * Remove a Hide rectangle: the eraser tapped on it.
   * @param {number} index
   * @returns {boolean}
   */
  removeHide(index) {
    const page = this.page;
    const hide = page.hides[index];
    if (!this.mayMakePages() || !hide) return false;
    page.hides.splice(index, 1);
    page.imagesRevision += 1;
    this.sendPageNow();
    this.events.onImagesChanged();
    this.extentChanged(true);
    this.pushUndo({ kind: 'hide', id: hide.id, before: { ...hide } });
    return true;
  }

  /**
   * Let the sheet hold a rectangle with the image margin around it.
   * @param {import('./page_model.js').Rectangle} rect
   */
  sheetAround(rect) {
    const page = this.page;
    const sheet = sheetHolding(page, rect);
    page.baseLeft = sheet.left;
    page.baseTop = sheet.top;
    page.baseWidth = sheet.width;
    page.baseHeight = sheet.height;
  }

  /**
   * Bring an image to the front, among the images.
   * @param {number} index
   * @returns {number}  Its index now.
   */
  raiseImage(index) {
    const page = this.page;
    if (!this.mayMakePages() || index < 0 || index >= page.images.length) return index;
    if (index !== page.images.length - 1) {
      const [image] = page.images.splice(index, 1);
      page.images.push(image);
      this.imagesChangedByHand();
    }
    return page.images.length - 1;
  }

  /**
   * Give an image a new rectangle: moved or scaled by hand (Andy's
   * teacher, 2026-10-08).  Writing stays where it is.  The image comes
   * to the front, the sheet grows to hold it, and everyone sees it move.
   * @param {number} index
   * @param {import('./page_model.js').Rectangle} rect
   * @returns {number}  The image's index now.
   */
  placeImageByHand(index, rect) {
    const page = this.page;
    if (!this.mayMakePages() || index < 0 || index >= page.images.length) return index;
    index = this.raiseImage(index);
    const image = page.images[index];
    image.x = Math.round(rect.left);
    image.y = Math.round(rect.top);
    image.width = Math.round(rect.width);
    image.height = Math.round(rect.height);
    this.sheetAround({ left: image.x, top: image.y, width: image.width, height: image.height });
    this.imagesChangedByHand();
    return index;
  }

  /** An image was raised, moved or scaled: tell everyone, soon. */
  imagesChangedByHand() {
    const page = this.page;
    page.imagesRevision += 1;
    this.sendPageSoon();
    this.events.onImagesChanged();
    this.extentChanged(true);
  }

  /**
   * Send the page descriptor, at most every few tens of milliseconds
   * while an image is being dragged; sendPageNow() at the end.
   */
  sendPageSoon() {
    if (this.pageSendTimer !== null) return;
    this.pageSendTimer = setTimeout(() => {
      this.pageSendTimer = null;
      this.send(0, { type: 'page', page: this.page.descriptor() });
    }, PAGE_SEND_INTERVAL_MS);
  }

  sendPageNow() {
    if (this.pageSendTimer !== null) {
      clearTimeout(this.pageSendTimer);
      this.pageSendTimer = null;
    }
    this.send(0, { type: 'page', page: this.page.descriptor() });
  }

  /**
   * Make the sheet fill the window, for more room to write (Andy,
   * 2026-10-08 as "Widen", which only grew the width; both ways since
   * 2026-10-11, since there is no reason width alone suits anyone).
   * Everyone's page grows; it is a change to the page, so it needs the
   * right to make pages, and it ends the chance to take back an image.
   * @param {number} width   The page's new width, if wider than now.
   * @param {number} height  The page's new height, if taller than now.
   * @returns {boolean}  Whether the page grew.
   */
  expandPage(width, height) {
    const page = this.page;
    if (!this.mayMakePages() || page.number === 0) return false;
    // Equally on both sides, and top and bottom, of the PAGE as it is,
    // writing included (the sheet may lie inside a page that writing has
    // grown; measured from the sheet, the page came out short, Andy,
    // 2026-10-08): what is on the page stays where it is seen.  The same
    // whole number of units on each side, since the page is centred on
    // the screen: an odd growth split 1199 and 1200 moved the centre half
    // a unit, a pixel's fraction that showed as a one-pixel move on Olion
    // (2026-10-09).  A dimension already filling the window does not grow.
    const addLeft = Math.floor((Math.round(width) - page.width) / 2);
    const addTop = Math.floor((Math.round(height) - page.height) / 2);
    if (addLeft <= 0 && addTop <= 0) return false;
    if (addLeft > 0) {
      page.baseLeft = page.left - addLeft;
      page.baseWidth = page.width + 2 * addLeft;
    }
    if (addTop > 0) {
      page.baseTop = page.top - addTop;
      page.baseHeight = page.height + 2 * addTop;
    }
    page.imagesRevision += 1;
    this.send(0, { type: 'page', page: page.descriptor() });
    this.extentChanged(true);
    return true;
  }

  /**
   * Make the sheet the smallest that holds everything on the page with
   * the margin around it ("Shrink", Andy, 2026-10-08).  A change to the
   * page like "Expand".
   * @returns {boolean}  Whether the sheet changed.
   */
  shrinkPage() {
    const page = this.page;
    if (!this.mayMakePages() || page.number === 0) return false;
    const sheet = page.contentSheet();
    if (!sheet || (sheet.left === page.baseLeft && sheet.top === page.baseTop &&
                   sheet.width === page.baseWidth && sheet.height === page.baseHeight)) return false;
    page.baseLeft = sheet.left;
    page.baseTop = sheet.top;
    page.baseWidth = sheet.width;
    page.baseHeight = sheet.height;
    page.imagesRevision += 1;
    this.send(0, { type: 'page', page: page.descriptor() });
    this.extentChanged(true);
    return true;
  }

  /**
   * Remove an image: the eraser tapped on it (Andy, 2026-10-08: "the user
   * has decided that the image should be deleted").  Allowed to whoever
   * may make pages, and to whoever added the image.  Writing on it stays
   * where it is; the sheet keeps its size ("Shrink" trims it).
   * @param {number} index
   * @returns {boolean}  Whether it was removed.
   */
  removeImage(index) {
    const page = this.page;
    const image = page.images[index];
    if (!image || !(this.mayMakePages() || image.by === this.identity.id)) return false;
    page.images.splice(index, 1);
    page.imagesRevision += 1;
    this.imageTransfers.delete(image.hash);
    this.sendPageNow();
    this.events.onImagesChanged();
    this.extentChanged(true);
    return true;
  }

  /**
   * Start a new page for everyone from a page kept in a notebook, with
   * everything that was on it.  Its layers keep their owners: a layer of
   * someone present is theirs to erase again.
   * @param {Page} kept  As read from the page file.
   */
  loadPage(kept) {
    if (!this.mayMakePages()) return;
    const page = new Page(kept.baseWidth, kept.baseHeight, randomId(), this.page.number + 1);
    page.baseLeft = kept.baseLeft;
    page.baseTop = kept.baseTop;
    page.images = kept.images.map((image) => ({ ...image }));
    page.hides = kept.hides.map((hide) => ({ ...hide }));
    page.imagesRevision = kept.imagesRevision;
    for (const [id, layer] of kept.layers) {
      const copy = page.layer(id, layer.name);
      for (const stroke of layer.strokes.values()) {
        copy.strokes.set(stroke.id, { ...stroke, points: stroke.points.slice(), done: true });
      }
      for (const block of layer.texts.values()) copy.texts.set(block.id, { ...block, lines: block.lines.slice() });
      if (!this.participants.has(id)) {
        this.participants.set(id, {
          id, name: layer.name || 'Unnamed', peer: null, self: false, host: id === this.hostId,
        });
      }
    }
    page.recomputeStrokeBounds();
    page.updateExtent();
    this.replacePage(page);
    this.send(0, { type: 'page', page: page.descriptor() });
    const sent = new Set();
    for (const image of page.images) {
      if (!sent.has(image.hash)) this.sendImage(0, image);
      sent.add(image.hash);
    }
    for (const layer of page.layers.values()) {
      if (!layer.isEmpty()) this.sendLayer(0, layer);
    }
    this.events.onParticipantsChanged();
  }

  // Text on our own layer.

  /**
   * Make or change a text block of ours.  The whole block is sent each
   * time, so that it appears on the other screens as it is typed.
   * @param {TextBlock} block
   */
  setText(block) {
    if (this.page.number === 0) return;
    this.ownLayer.texts.set(block.id, block);
    this.send(0, { type: 'text_set', page: this.page.id, text: block });
    this.page.recomputeStrokeBounds();
    this.events.onTextChanged(this.identity.id, block);
    this.extentChanged();
  }

  /**
   * A text block of ours is about to be edited, as a whole gesture: typed
   * into until the editing ends, or moved by hand until the pen lifts.
   * The block as it is now is what undo restores; null for a new block.
   * Only one gesture at a time: a hold on the block being typed in is part
   * of the typing (Andy, 2026-10-09: a hold on the field moves the block).
   * @param {string} id
   * @param {string} owner  Who began it: 'editor' or 'hold'.
   * @returns {boolean}  Whether this call began the gesture.
   */
  beginTextEdit(id, owner) {
    if (this.textEditing) return false;
    const block = this.ownLayer.texts.get(id);
    this.textEditing = { id, owner, before: block ? copyText(block) : null };
    return true;
  }

  /**
   * The gesture ended: record it if the block changed.
   * @param {string} id
   * @param {string} owner
   */
  endTextEdit(id, owner) {
    const editing = this.textEditing;
    if (!editing || editing.id !== id || editing.owner !== owner) return;
    this.textEditing = null;
    const now = this.ownLayer.texts.get(id) ?? null;
    if (JSON.stringify(now) !== JSON.stringify(editing.before)) {
      this.pushUndo({ kind: 'text', id, before: editing.before });
    }
  }

  /** @param {string[]} ids  Text blocks of our own layer. */
  removeTexts(ids) {
    const removedBlocks = ids.map((id) => this.ownLayer.texts.get(id)).filter((block) => block !== undefined);
    const removed = ids.filter((id) => this.ownLayer.texts.delete(id));
    if (removed.length === 0) return;
    if (this.erasing) this.erasing.texts.push(...removedBlocks);
    this.send(0, { type: 'text_remove', page: this.page.id, ids: removed });
    this.page.recomputeStrokeBounds();
    this.events.onLayerReplaced(this.identity.id);
    this.extentChanged();
  }

  /**
   * @param {number} destination
   * @param {PageImage} image  An image of the present page whose bytes we hold.
   */
  sendImage(destination, image) {
    const data = image.data;
    if (!data) return;
    for (let at = 0; at < data.length; at += IMAGE_CHUNK_BYTES) {
      this.send(destination, {
        type: 'image', page: this.page.id, hash: image.hash, at,
        data: toBase64(data.subarray(at, at + IMAGE_CHUNK_BYTES)),
      });
    }
  }

  /**
   * Ask a peer that is on our page for the images whose bytes we lack.
   * @param {number} peer
   */
  requestImages(peer) {
    const asked = new Set();
    for (const image of this.page.images) {
      if (image.data || asked.has(image.hash)) continue;
      asked.add(image.hash);
      this.send(peer, { type: 'image_request', page: this.page.id, hash: image.hash });
    }
  }

  /**
   * A piece of one of the page's images arrived.  Pieces are taken in
   * sequence; the finished image must match its hash in the page
   * descriptor, so it does not matter which participant sent it.
   * @param {any} message
   */
  async receivedImagePiece(message) {
    const page = this.page;
    if (message.page !== page.id || typeof message.data !== 'string') return;
    const image = page.images.find((one) => one.hash === message.hash && !one.data);
    if (!image) return;
    let transfer = this.imageTransfers.get(image.hash);
    if (!transfer) {
      transfer = { buffer: new Uint8Array(image.size), received: 0 };
      this.imageTransfers.set(image.hash, transfer);
    }
    if (message.at !== transfer.received) return;
    const piece = fromBase64(message.data);
    if (transfer.received + piece.length > image.size) return;
    transfer.buffer.set(piece, transfer.received);
    transfer.received += piece.length;
    if (transfer.received < image.size) return;

    this.imageTransfers.delete(image.hash);
    const data = transfer.buffer;
    const matches = await hashOf(data) === image.hash;
    if (page !== this.page || !matches) return;     // A damaged or false image: wait for another copy.
    let filled = false;
    for (const one of page.images) {
      if (one.hash !== image.hash || one.data) continue;
      one.data = data;
      filled = true;
    }
    if (!filled) return;
    this.events.onImagesChanged();
    for (const peer of this.imageRequests.get(image.hash) ?? []) this.sendImage(peer, image);
    this.imageRequests.delete(image.hash);
  }

  /**
   * Move to a page that another participant is on, if it is newer than
   * ours.  Pages are numbered by the host, so the highest number is the
   * session's current page; between two pages of the same number, the
   * host's is taken.  If it is the page we are on, it may have gained
   * images.
   * @param {unknown} received  A page descriptor.
   * @param {boolean} fromHost
   * @param {number} peer  Who told us.
   */
  considerPage(received, fromHost, peer) {
    const descriptor = cleanPageDescriptor(received);
    if (!descriptor) return;
    if (descriptor.id === this.page.id) {
      this.considerImages(descriptor);
      return;
    }
    const newer = descriptor.number > this.page.number ||
      (descriptor.number === this.page.number && fromHost);
    if (!newer) return;
    this.replacePage(Page.from(descriptor));
    this.pageSourcePeer = peer;
  }

  /**
   * Whether a page descriptor from a participant without the right to
   * make pages is one they may still send: our page, the sheet as it is,
   * and the images ours with only images they added left out (Andy,
   * 2026-10-08: whoever added an image may remove it, right or no right).
   * @param {any} received
   * @param {string} id  The sender.
   * @returns {boolean}
   */
  removesOnlyOwnImages(received, id) {
    const descriptor = cleanPageDescriptor(received);
    const page = this.page;
    if (!descriptor || descriptor.id !== page.id || descriptor.left !== page.baseLeft ||
        descriptor.top !== page.baseTop || descriptor.width !== page.baseWidth ||
        descriptor.height !== page.baseHeight ||
        JSON.stringify(descriptor.hides) !== JSON.stringify(page.hides)) return false;
    /** @param {Omit<PageImage, 'data'>} image */
    const key = (image) => `${image.hash} ${image.x} ${image.y} ${image.width} ${image.height} ${image.by}`;
    let at = 0;
    let removed = 0;
    for (const ours of page.images) {
      if (at < descriptor.images.length && key(descriptor.images[at]) === key(ours)) {
        at += 1;
      } else if (ours.by === id) {
        removed += 1;
      } else {
        return false;
      }
    }
    return at === descriptor.images.length && removed > 0;
  }

  /**
   * Another participant's list of the images on the page we are on.  The
   * list with the higher revision is the later one.  Of two different
   * lists of the same revision (two participants changed the images at the
   * same moment) everyone takes the same one, and the change that lost
   * must be made again.
   * @param {import('./page_model.js').PageDescriptor} descriptor
   */
  considerImages(descriptor) {
    const page = this.page;
    /** @param {import('./page_model.js').PageDescriptor} d */
    const listed = (d) => d.images.map((image) =>
      `${image.hash} ${image.x} ${image.y} ${image.width} ${image.height}`).join(',') +
      '|' + d.hides.map((hide) => `${hide.id} ${hide.x} ${hide.y} ${hide.width} ${hide.height}`).join(',') +
      `|${d.left} ${d.top} ${d.width} ${d.height}`;
    const ours = listed(page.descriptor());
    const theirs = listed(descriptor);
    if (descriptor.revision < page.imagesRevision ||
        (descriptor.revision === page.imagesRevision && theirs <= ours)) return;
    const held = new Map();
    for (const image of page.images) {
      if (image.data) held.set(image.hash, image.data);
    }
    page.images = descriptor.images.map((image) => ({ ...image, data: held.get(image.hash) ?? null }));
    page.hides = descriptor.hides.map((hide) => ({ ...hide }));
    page.imagesRevision = descriptor.revision;
    page.baseLeft = descriptor.left;
    page.baseTop = descriptor.top;
    page.baseWidth = descriptor.width;
    page.baseHeight = descriptor.height;
    // Bytes on their way for an image that is no longer wanted are dropped,
    // so that the same image, added again, is received from its beginning.
    for (const hash of this.imageTransfers.keys()) {
      if (!page.images.some((image) => image.hash === hash && !image.data)) this.imageTransfers.delete(hash);
    }
    this.events.onImagesChanged();
    this.extentChanged(true);
  }

  /** @param {Page} page */
  replacePage(page) {
    this.endStroke();
    const old = this.page;
    this.page = page;
    this.undoStack = [];
    this.erasing = null;
    this.textEditing = null;
    this.hideEditing = null;
    this.events.onUndoChanged?.();
    this.ownLayer = page.layer(this.identity.id, this.name);
    for (const participant of this.participants.values()) {
      if (participant.peer !== null) page.layer(participant.id, participant.name);
    }
    this.incoming.clear();
    this.imageTransfers.clear();
    this.imageRequests.clear();
    this.pageSourcePeer = 0;
    this.events.onPageReplaced(old, page);
  }

  // Drawing on our own layer.

  /**
   * @param {string} color
   * @param {number} width
   * @returns {Stroke}
   */
  beginStroke(color, width) {
    if (this.currentStroke) this.endStroke();
    const stroke = { id: randomId(), color, width, points: [], done: false };
    this.ownLayer.strokes.set(stroke.id, stroke);
    this.currentStroke = stroke;
    this.send(0, {
      type: 'stroke_begin', page: this.page.id, stroke: { id: stroke.id, color, width },
    });
    return stroke;
  }

  /** @param {number[]} points  x, y, pressure for each point. */
  addPoints(points) {
    let stroke = this.currentStroke;
    if (!stroke || points.length === 0) return;
    if (pointCount(stroke) + points.length / POINT_SIZE > MAX_STROKE_POINTS) {
      const last = stroke.points.slice(-POINT_SIZE);
      this.endStroke();
      stroke = this.beginStroke(stroke.color, stroke.width);
      points = last.concat(points);
    }
    const at = pointCount(stroke);
    appendPoints(stroke, at, points);
    this.page.includeStroke(stroke, at);
    this.queuePoints(stroke.id, at, points);
    this.events.onStrokeProgress(this.identity.id, stroke);
    this.extentChanged();
  }

  /**
   * Points to send: at once if none went in the last interval, else with
   * the next sending, which the timer makes when the interval is up.
   * @param {string} id
   * @param {number} at
   * @param {number[]} points
   */
  queuePoints(id, at, points) {
    if (this.pendingPoints && this.pendingPoints.id === id) {
      this.pendingPoints.points.push(...points);
    } else {
      this.sendPendingPoints();
      this.pendingPoints = { id, at, points: points.slice() };
    }
    const since = performance.now() - this.lastPointSend;
    if (since >= POINT_SEND_INTERVAL_MS) {
      this.sendPendingPoints();
    } else if (this.pointSendTimer === null) {
      this.pointSendTimer = setTimeout(() => {
        this.pointSendTimer = null;
        this.sendPendingPoints();
      }, POINT_SEND_INTERVAL_MS - since);
    }
  }

  sendPendingPoints() {
    if (this.pointSendTimer !== null) {
      clearTimeout(this.pointSendTimer);
      this.pointSendTimer = null;
    }
    const pending = this.pendingPoints;
    if (!pending) return;
    this.pendingPoints = null;
    this.lastPointSend = performance.now();
    this.send(0, { type: 'stroke_points', id: pending.id, at: pending.at, points: pending.points });
  }

  /**
   * @param {boolean} [record]  Whether the stroke is a gesture to undo;
   *     not when it is about to be dropped, as a hold's beginning is.
   */
  endStroke(record = true) {
    const stroke = this.currentStroke;
    if (!stroke) return;
    this.currentStroke = null;
    stroke.done = true;
    this.sendPendingPoints();
    this.send(0, { type: 'stroke_end', id: stroke.id });
    this.events.onStrokeProgress(this.identity.id, stroke);
    if (record && pointCount(stroke) > 0) this.pushUndo({ kind: 'stroke', id: stroke.id });
  }

  /**
   * The eraser went down: what it removes until it lifts is one gesture.
   * Harmless when already begun.
   */
  beginErasing() {
    if (!this.erasing) this.erasing = { strokes: [], texts: [] };
  }

  /**
   * The eraser lifted: the gesture is recorded if it removed anything.
   * @returns {{strokes: number, texts: number}}  What it removed.
   */
  endErasing() {
    const erasing = this.erasing;
    this.erasing = null;
    if (!erasing) return { strokes: 0, texts: 0 };
    if (erasing.strokes.length || erasing.texts.length) {
      this.pushUndo({ kind: 'erase', strokes: erasing.strokes, texts: erasing.texts });
    }
    return { strokes: erasing.strokes.length, texts: erasing.texts.length };
  }

  /** @param {string[]} ids  Strokes of our own layer. */
  erase(ids) {
    const erasedStrokes = ids.map((id) => this.ownLayer.strokes.get(id)).filter((stroke) => stroke !== undefined);
    const erased = ids.filter((id) => this.ownLayer.strokes.delete(id));
    if (erased.length === 0) return;
    if (this.erasing) this.erasing.strokes.push(...erasedStrokes);
    this.send(0, { type: 'erase', page: this.page.id, ids: erased });
    this.page.recomputeStrokeBounds();
    this.events.onLayerReplaced(this.identity.id);
    this.extentChanged();
  }

  /**
   * Put erased strokes of ours back, whole, for everyone.
   * @param {Stroke[]} strokes
   */
  restoreStrokes(strokes) {
    for (const stroke of strokes) this.ownLayer.strokes.set(stroke.id, stroke);
    /** @type {Stroke[]} */
    let chunk = [];
    let size = 0;
    for (const stroke of strokes) {
      chunk.push(stroke);
      size += 100 + stroke.points.length * 8;
      if (size >= SNAPSHOT_CHUNK_CHARACTERS) {
        this.send(0, { type: 'stroke_restore', page: this.page.id, strokes: chunk });
        chunk = [];
        size = 0;
      }
    }
    if (chunk.length) this.send(0, { type: 'stroke_restore', page: this.page.id, strokes: chunk });
    this.page.recomputeStrokeBounds();
    this.events.onLayerReplaced(this.identity.id);
    this.extentChanged();
  }

  // Undo: one's own gestures, the latest first (Andy, 2026-10-11).

  /** @param {UndoEntry} entry */
  pushUndo(entry) {
    this.undoStack.push(entry);
    if (this.undoStack.length > UNDO_LIMIT) this.undoStack.shift();
    this.events.onUndoChanged?.();
  }

  canUndo() {
    return this.undoStack.length > 0;
  }

  /**
   * Take back our latest gesture.
   * @returns {string | null}  What was undone, for the log; null if nothing.
   */
  undo() {
    const entry = this.undoStack.pop();
    if (!entry) return null;
    this.events.onUndoChanged?.();
    switch (entry.kind) {
      case 'stroke':
        this.erase([entry.id]);
        return 'a stroke';
      case 'erase':
        this.restoreStrokes(entry.strokes);
        for (const block of entry.texts) this.setText(block);
        return `an erasure of ${entry.strokes.length} strokes and ${entry.texts.length} texts`;
      case 'text':
        if (entry.before) this.setText(entry.before);
        else this.removeTexts([entry.id]);
        return entry.before ? 'a change of text' : 'a text block';
      case 'hide': {
        const page = this.page;
        if (!this.mayMakePages()) return null;
        const index = page.hides.findIndex((one) => one.id === entry.id);
        if (entry.before === null) {
          if (index < 0) return null;
          page.hides.splice(index, 1);
        } else if (index < 0) {
          page.hides.push({ ...entry.before });
          this.sheetAround(rectOf(entry.before));
        } else {
          Object.assign(page.hides[index], entry.before);
          this.sheetAround(rectOf(entry.before));
        }
        page.imagesRevision += 1;
        this.sendPageNow();
        this.events.onImagesChanged();
        this.extentChanged(true);
        return entry.before === null ? 'a Hide rectangle' : index < 0 ? 'the removal of a Hide rectangle' : 'a move of a Hide rectangle';
      }
      default:
        return null;
    }
  }

  // Sending.

  /**
   * Queue a message.  Messages are encrypted and sent in the sequence in
   * which they were queued.
   * @param {number} destination  A peer number, or 0 for everyone else.
   * @param {object | (() => Promise<object>)} message
   */
  send(destination, message) {
    if (!this.connection.connected) return;
    const epoch = this.epoch;
    this.sendChain = this.sendChain.then(async () => {
      if (epoch !== this.epoch) return;
      const body = typeof message === 'function' ? await message() : message;
      const bytes = await encrypt(this.key, { ...body, seq: this.seq++ });
      if (epoch === this.epoch) this.connection.send(destination, bytes);
    }).catch((error) => console.error('Kokkoja: send failed', error));
  }

  /**
   * Send a whole layer.  The receiver replaces its copy of the layer.
   * @param {number} destination
   * @param {Layer} layer
   */
  sendLayer(destination, layer) {
    this.send(destination, {
      type: 'layer_begin', page: this.page.id, layer: layer.id, name: layer.name,
    });
    /** @type {Stroke[]} */
    let chunk = [];
    let size = 0;
    for (const stroke of layer.strokes.values()) {
      chunk.push(stroke);
      size += 100 + stroke.points.length * 8;
      if (size >= SNAPSHOT_CHUNK_CHARACTERS) {
        this.send(destination, { type: 'layer_strokes', layer: layer.id, strokes: chunk });
        chunk = [];
        size = 0;
      }
    }
    if (chunk.length) this.send(destination, { type: 'layer_strokes', layer: layer.id, strokes: chunk });
    if (layer.texts.size) {
      this.send(destination, { type: 'layer_texts', layer: layer.id, texts: [...layer.texts.values()] });
    }
    this.send(destination, { type: 'layer_end', layer: layer.id });
  }

  /**
   * Announce ourselves, with proof that this connection is ours.
   * @param {number} destination
   * @param {boolean} reply  True when answering another participant's hello.
   */
  sendHello(destination, reply) {
    const peer = this.myPeer;
    this.send(destination, async () => ({
      type: 'hello', reply, name: this.name, publicKey: this.identity.publicKey,
      signature: await signClaim(this.identity, this.roomId, peer),
      page: this.page.number > 0 ? this.page.descriptor() : null,
      permitted: this.isHost ? [...this.permitted] : undefined,
    }));
  }

  // Connection events.

  /**
   * @param {number} peer
   * @param {number[]} peers
   */
  welcomed(peer, peers) {
    this.epoch += 1;
    this.myPeer = peer;
    this.presentPeers = new Set(peers);
    const self = this.participants.get(this.identity.id);
    if (self) self.peer = peer;
    this.recoverOwnLayer = this.firstConnection && peers.length > 0;
    this.firstConnection = false;

    this.sendHello(0, false);
    // While recovering, our layer is sent when the recovery is complete.
    if (!this.recoverOwnLayer && !this.ownLayer.isEmpty()) this.sendLayer(0, this.ownLayer);
    this.requestState();
    this.events.onParticipantsChanged();
  }

  /** Ask the longest-connected peer for the layers of absent participants. */
  requestState() {
    this.stateComplete = this.presentPeers.size === 0;
    if (this.stateComplete) this.recoverOwnLayer = false;
    this.statePeer = this.stateComplete ? 0 : Math.min(...this.presentPeers);
    if (this.statePeer) this.send(this.statePeer, { type: 'state_request' });
  }

  disconnected() {
    this.epoch += 1;
    this.peerIds.clear();
    this.presentPeers.clear();
    this.lastSeq.clear();
    this.incoming.clear();
    for (const participant of this.participants.values()) participant.peer = null;
    this.finishStrokesOfAbsent();
    this.events.onParticipantsChanged();
  }

  /** @param {number} peer */
  peerLeft(peer) {
    this.presentPeers.delete(peer);
    this.lastSeq.delete(peer);
    const id = this.peerIds.get(peer);
    this.peerIds.delete(peer);
    const participant = id ? this.participants.get(id) : undefined;
    if (participant && participant.peer === peer) participant.peer = null;
    this.finishStrokesOfAbsent();
    if (peer === this.statePeer && !this.stateComplete) this.requestState();
    this.events.onParticipantsChanged();
  }

  /** A stroke whose writer has gone will get no more points. */
  finishStrokesOfAbsent() {
    for (const layer of this.page.layers.values()) {
      const owner = this.participants.get(layer.id);
      if (owner && (owner.self || owner.peer !== null)) continue;
      for (const stroke of layer.strokes.values()) {
        if (stroke.done) continue;
        stroke.done = true;
        this.events.onStrokeProgress(layer.id, stroke);
      }
    }
  }

  // Receiving.

  /**
   * @param {number} peer
   * @param {Uint8Array<ArrayBuffer>} body
   */
  received(peer, body) {
    const epoch = this.epoch;
    this.receiveChain = this.receiveChain.then(async () => {
      if (epoch !== this.epoch) return;
      let message;
      try {
        message = await decrypt(this.key, body);
      } catch {
        return;     // Not made with the session key.
      }
      if (epoch !== this.epoch || !message || typeof message.seq !== 'number') return;
      if (message.seq <= (this.lastSeq.get(peer) ?? -1)) return;
      this.lastSeq.set(peer, message.seq);
      if (message.type === 'hello') await this.receivedHello(peer, message, epoch);
      else this.receivedFromParticipant(peer, message);
    }).catch((error) => console.error('Kokkoja: receive failed', error));
  }

  /**
   * @param {number} peer
   * @param {any} message
   * @param {number} epoch
   */
  async receivedHello(peer, message, epoch) {
    const id = await verifyClaim(message.publicKey, message.signature, this.roomId, peer);
    if (epoch !== this.epoch || !id || id === this.identity.id) return;

    const name = cleanName(message.name) || 'Unnamed';
    const previous = this.participants.get(id);
    if (previous && previous.peer !== null) this.peerIds.delete(previous.peer);
    this.peerIds.set(peer, id);
    this.participants.set(id, { id, name, peer, self: false, host: id === this.hostId });
    if (id === this.hostId) this.receivedPermissions(message.permitted);
    this.considerPage(message.page, id === this.hostId, peer);
    // This peer is on our page: it can give us the images we lack.
    if (message.page?.id === this.page.id) this.requestImages(peer);
    this.page.layer(id, name);
    this.events.onParticipantsChanged();

    if (!message.reply) {
      this.sendHello(peer, true);
      if (!this.ownLayer.isEmpty()) this.sendLayer(peer, this.ownLayer);
    }
  }

  /**
   * A message other than hello.  It is accepted only from a peer that has
   * proved who it is, and changes only that participant's layer.
   * @param {number} peer
   * @param {any} message
   */
  receivedFromParticipant(peer, message) {
    const id = this.peerIds.get(peer);
    if (!id) return;
    if (message.type === 'page') {
      if (id === this.hostId || this.permitted.has(id)) this.considerPage(message.page, true, peer);
      else if (this.removesOnlyOwnImages(message.page, id)) this.considerPage(message.page, false, peer);
      return;
    }
    if (message.type === 'permissions') {
      if (id === this.hostId) this.receivedPermissions(message.ids);
      return;
    }
    if (message.type === 'view') {
      if (id === this.hostId && (message.command === 'fit' || message.command === 'sheet')) {
        this.events.onViewCommand(message.command);
      }
      return;
    }
    if (message.type === 'image') {
      this.receivedImagePiece(message);
      return;
    }
    if (message.type === 'image_request') {
      if (message.page !== this.page.id) return;
      const image = this.page.images.find((one) => one.hash === message.hash);
      if (!image) return;
      if (image.data) {
        this.sendImage(peer, image);
      } else {
        const waiting = this.imageRequests.get(image.hash) ?? new Set();
        waiting.add(peer);
        this.imageRequests.set(image.hash, waiting);
      }
      return;
    }
    const layer = this.page.layer(id);

    switch (message.type) {
      case 'stroke_begin': {
        if (message.page !== this.page.id) break;
        const stroke = cleanStroke({ ...message.stroke, points: [], done: false });
        if (stroke) layer.strokes.set(stroke.id, stroke);
        break;
      }
      case 'stroke_points': {
        const stroke = layer.strokes.get(message.id);
        if (!stroke || stroke.done || !validPoints(message.points) ||
            !Number.isInteger(message.at) || message.at < 0) break;
        const before = pointCount(stroke);
        appendPoints(stroke, message.at, message.points);
        if (pointCount(stroke) > MAX_STROKE_POINTS) stroke.points.length = MAX_STROKE_POINTS * POINT_SIZE;
        this.page.includeStroke(stroke, before);
        this.events.onStrokeProgress(id, stroke);
        this.extentChanged();
        break;
      }
      case 'stroke_end': {
        const stroke = layer.strokes.get(message.id);
        if (!stroke || stroke.done) break;
        stroke.done = true;
        this.events.onStrokeProgress(id, stroke);
        break;
      }
      case 'stroke_restore': {
        if (message.page !== this.page.id || !Array.isArray(message.strokes)) break;
        let restored = false;
        for (const received of message.strokes) {
          const stroke = cleanStroke({ ...received, done: true });
          if (stroke) {
            layer.strokes.set(stroke.id, stroke);
            restored = true;
          }
        }
        if (!restored) break;
        this.page.recomputeStrokeBounds();
        this.events.onLayerReplaced(id);
        this.extentChanged();
        break;
      }
      case 'erase': {
        if (message.page !== this.page.id || !Array.isArray(message.ids)) break;
        let erased = false;
        for (const strokeId of message.ids) erased = layer.strokes.delete(strokeId) || erased;
        if (!erased) break;
        this.page.recomputeStrokeBounds();
        this.events.onLayerReplaced(id);
        this.extentChanged();
        break;
      }
      case 'text_set': {
        if (message.page !== this.page.id) break;
        const block = cleanText(message.text);
        if (!block) break;
        layer.texts.set(block.id, block);
        this.page.recomputeStrokeBounds();
        this.events.onTextChanged(id, block);
        this.extentChanged();
        break;
      }
      case 'text_remove': {
        if (message.page !== this.page.id || !Array.isArray(message.ids)) break;
        let removed = false;
        for (const textId of message.ids) removed = layer.texts.delete(textId) || removed;
        if (!removed) break;
        this.page.recomputeStrokeBounds();
        this.events.onLayerReplaced(id);
        this.extentChanged();
        break;
      }
      case 'layer_begin':
        if (message.page === this.page.id && validId(message.layer) &&
            this.acceptsLayerFrom(peer, id, message.layer)) {
          this.incoming.set(`${peer} ${message.layer}`,
                            { name: cleanName(message.name), strokes: [], texts: [] });
        }
        break;
      case 'layer_texts': {
        const snapshot = this.incoming.get(`${peer} ${message.layer}`);
        if (!snapshot || !Array.isArray(message.texts)) break;
        for (const received of message.texts) {
          const block = cleanText(received);
          if (block) snapshot.texts.push(block);
        }
        break;
      }
      case 'layer_strokes': {
        const snapshot = this.incoming.get(`${peer} ${message.layer}`);
        if (!snapshot || !Array.isArray(message.strokes)) break;
        for (const received of message.strokes) {
          const stroke = cleanStroke(received);
          if (stroke) snapshot.strokes.push(stroke);
        }
        break;
      }
      case 'layer_end': {
        const key = `${peer} ${message.layer}`;
        const snapshot = this.incoming.get(key);
        this.incoming.delete(key);
        if (snapshot && this.acceptsLayerFrom(peer, id, message.layer)) {
          this.applySnapshot(message.layer, snapshot, message.layer === id);
        }
        break;
      }
      case 'state_request':
        this.sendState(peer, id);
        break;
      case 'state_end':
        if (peer !== this.statePeer) break;
        this.stateComplete = true;
        if (this.recoverOwnLayer) {
          this.recoverOwnLayer = false;
          if (!this.ownLayer.isEmpty()) this.sendLayer(0, this.ownLayer);
        }
        break;
    }
  }

  /**
   * The host said who may start pages.
   * @param {unknown} ids
   */
  receivedPermissions(ids) {
    if (!Array.isArray(ids)) return;
    const permitted = new Set(ids.filter(validId));
    const same = permitted.size === this.permitted.size && [...permitted].every((id) => this.permitted.has(id));
    if (same) return;
    this.permitted = permitted;
    this.events.onPermissionsChanged();
  }

  /**
   * Whether to accept a snapshot of a layer from a peer.  A participant may
   * always send their own layer.  The peer that moved us to this page may
   * send any of its layers, our own included while we have nothing on it
   * (a page loaded from a notebook comes with its writing).  The peer we
   * asked for the session state may send the layers of participants who
   * are not connected, and our own layer while we are recovering it (the
   * browser page was reloaded).
   * @param {number} peer
   * @param {string} senderId
   * @param {string} layerId
   */
  acceptsLayerFrom(peer, senderId, layerId) {
    if (layerId === senderId) return true;
    if (peer === this.pageSourcePeer) {
      return layerId !== this.identity.id || this.ownLayer.isEmpty();
    }
    if (peer !== this.statePeer) return false;
    if (layerId === this.identity.id) return this.recoverOwnLayer;
    const owner = this.participants.get(layerId);
    return !owner || owner.peer === null;
  }

  /**
   * @param {string} layerId
   * @param {{name: string, strokes: Stroke[], texts: TextBlock[]}} snapshot
   * @param {boolean} fromOwner
   */
  applySnapshot(layerId, snapshot, fromOwner) {
    const layer = this.page.layer(layerId, snapshot.name);
    if (layerId === this.identity.id) {
      // Recovering our own layer: keep anything drawn since we joined.
      for (const stroke of snapshot.strokes) {
        if (!layer.strokes.has(stroke.id)) layer.strokes.set(stroke.id, { ...stroke, done: true });
      }
      for (const block of snapshot.texts) {
        if (!layer.texts.has(block.id)) layer.texts.set(block.id, block);
      }
    } else {
      layer.strokes.clear();
      for (const stroke of snapshot.strokes) {
        layer.strokes.set(stroke.id, fromOwner ? stroke : { ...stroke, done: true });
      }
      layer.texts.clear();
      for (const block of snapshot.texts) layer.texts.set(block.id, block);
      if (!this.participants.has(layerId)) {
        this.participants.set(layerId, {
          id: layerId, name: snapshot.name || 'Unnamed', peer: null, self: false,
          host: layerId === this.hostId,
        });
        this.events.onParticipantsChanged();
      }
    }
    this.page.recomputeStrokeBounds();
    this.events.onLayerReplaced(layerId);
    this.extentChanged();
  }

  /**
   * Answer a state request: send the layers that their owners cannot send
   * themselves.
   * @param {number} peer
   * @param {string} requesterId
   */
  sendState(peer, requesterId) {
    for (const layer of this.page.layers.values()) {
      if (layer.id === this.identity.id || layer.isEmpty()) continue;
      const owner = this.participants.get(layer.id);
      const ownerConnected = owner !== undefined && owner.peer !== null;
      if (layer.id === requesterId || !ownerConnected) this.sendLayer(peer, layer);
    }
    this.send(peer, { type: 'state_end' });
  }
}

/**
 * @param {TextBlock} block
 * @returns {TextBlock}
 */
function copyText(block) {
  return { ...block, lines: block.lines.slice() };
}

/**
 * @param {HideRect} a
 * @param {HideRect} b
 */
function sameRect(a, b) {
  return a.x === b.x && a.y === b.y && a.width === b.width && a.height === b.height;
}

/**
 * @param {HideRect} hide
 * @returns {import('./page_model.js').Rectangle}
 */
function rectOf(hide) {
  return { left: hide.x, top: hide.y, width: hide.width, height: hide.height };
}