Undo, and the toolbar in three groups

Undo takes back one's own latest mark, by the button, Ctrl+Z or two
quick taps of the eraser; the toolbar's drawing tools, the page and
view, and the session stand in three groups that do not move when the
tool changes; the buttons are a little smaller.

client/index.html

@@ -40,16 +40,14 @@
 
 <div id="session" hidden>
   <header class="toolbar">
+    <div id="drawing" class="drawing" aria-label="Drawing">
     <div class="group" role="radiogroup" aria-label="Tool">
       <button type="button" data-tool="pen" aria-pressed="true"
               data-help="Write on the page. Only your own writing can be erased.">Pen</button>
       <button type="button" data-tool="text" aria-pressed="false"
               data-help="Type on the page. Tap where the text should begin, or drag open a rectangle: a red box, white inside, in which the text wraps and which grows downward as you type. Tap your own text to change it; rest the pen on it to move it, or near a corner of a box to resize it.">Text</button>
-      <button type="button" data-tool="eraser" aria-pressed="false"
-              data-help="Erase your own strokes and text by moving over them. A tap on an image, or on a Hide rectangle, removes it, if you are the host or a co-host (or added the image yourself).">Eraser</button>
-      <button type="button" data-tool="hide" aria-pressed="false" hidden
-              data-help="Hide part of the page for everyone: drag open a rectangle, and what lies under it is covered in white, marked by blue corners. Rest the pen on it to move it, or near a corner to resize it; a tap of the eraser removes it.">Hide</button>
     </div>
+    <div id="settings" class="settings">
     <div class="group" id="colors" role="radiogroup" aria-label="Color">
       <button id="more_colors" type="button" class="swatch more"
               data-help="Choose another color. It is added here.">+</button>
@@ -65,7 +63,17 @@
       <input id="text_size" type="number" min="6" max="100" step="1" value="14"
              aria-label="Text size in points" data-help="Text size in points.">
     </div>
-    <div class="group" aria-label="View">
+    </div>
+    <div class="group after_settings" role="radiogroup" aria-label="Hide and Eraser">
+      <button type="button" data-tool="hide" aria-pressed="false" hidden
+              data-help="Hide part of the page for everyone: drag open a rectangle, and what lies under it is covered in white, marked by blue corners. Rest the pen on it to move it, or near a corner to resize it; a tap of the eraser removes it.">Hide</button>
+      <button type="button" data-tool="eraser" aria-pressed="false"
+              data-help="Erase your own strokes and text by moving over them. A tap on an image, or on a Hide rectangle, removes it, if you are the host or a co-host (or added the image yourself). Two quick taps on empty paper, away from anything the eraser would remove, undo your latest mark; again, and the one before.">Eraser</button>
+    </div>
+      <button id="undo" type="button" disabled
+              data-help="Take back your latest mark: a stroke, an erasure, a text block typed or moved, a Hide rectangle. Again for the one before. Also Ctrl+Z (Command+Z on a Mac), or two quick taps of the eraser on empty paper. Only your own marks, and only on this page since you opened it.">Undo</button>
+    </div>
+    <div class="group page_view" aria-label="Page and view">
       <button id="zoom_fit" type="button"
               data-help="Show the whole page in the window. Pressed by the host, it does so on every screen. Ctrl (Command on a Mac) with the scroll wheel, or with a pen stroke up or down, zooms about the pointer.">Fit</button>
       <button id="zoom_sheet" type="button"

client/js/main.js

@@ -33,7 +33,7 @@ import { writePicture } from './files.js';
 import { pageToPng } from './page_picture.js';
 import { TextEditor } from './text_editor.js';
 import { enableTooltips } from './tooltip.js';
-import { guardSelection } from './selection_guard.js';
+import { guardSelection, inField } from './selection_guard.js';
 import { QUIET_ZONE, qrModules, qrPath } from './qr_code.js';
 
 /**
@@ -56,6 +56,12 @@ 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
@@ -551,6 +557,9 @@ async function enterSession(invitationText, name, notebook) {
         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: ${text[status]}`);
@@ -616,16 +625,54 @@ async function enterSession(invitationText, name, notebook) {
   };
   new ResizeObserver(watchToolbar).observe(toolbar);
   watchToolbar();
+  // The drawing group keeps the width of its widest state, the pen's
+  // colours and widths, so that the page-and-view group, centred between
+  // it and the session group, stays put when the tool changes (Andy,
+  // 2026-10-11).  The floor rises if the group ever grows, as when a
+  // colour is added.
+  // The settings (colours, widths, font and size) keep their widest
+  // room likewise, so that Eraser, Hide and Undo, which stand after them
+  // since 2026-10-11, do not jump when a tool shows fewer settings.
+  for (const part of [element('drawing'), element('settings')]) {
+    let widest = 0;
+    new ResizeObserver(() => {
+      const width = part.getBoundingClientRect().width;
+      if (width > widest) {
+        widest = width;
+        part.style.minWidth = `${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 is finished first, so that the undo is of it.
+  const undo = () => {
+    textEditor?.stop();
+    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();
+    }
+  });
+
   // Whether one's own writing may grow the page (the "Extend" box).
   let extend = localStorage.getItem(EXTEND_KEY) !== 'no';
   const extendBox = input('extend');
@@ -671,6 +718,8 @@ async function enterSession(invitationText, name, notebook) {
    */
   /** @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;
 
@@ -765,6 +814,7 @@ async function enterSession(invitationText, name, notebook) {
       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);
@@ -773,6 +823,7 @@ async function enterSession(invitationText, name, notebook) {
           // 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 (session.mayMakePages()) {
           const image = page.imageAt(x, y, reach);
           if (image) {
@@ -786,7 +837,7 @@ async function enterSession(invitationText, name, notebook) {
       if (strokeInProgress) {
         const begun = strokeInProgress;
         strokeInProgress = null;
-        session.endStroke();
+        session.endStroke(false);
         session.erase([begun.id]);
       }
       textEditor?.hidePreview();
@@ -826,11 +877,14 @@ async function enterSession(invitationText, name, notebook) {
       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 !== 'text') session.sendPageNow();
+      if (held.kind === 'text') session.endTextEdit(held.id, 'hold');
+      if (held.kind === 'hide') session.endHideEdit();
       held = null;
       renderer.showGrip(null);
     },
     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);
@@ -839,16 +893,40 @@ async function enterSession(invitationText, name, notebook) {
       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
       // (the right to make pages, or having added the image).
-      if (reach >= TAP_DISTANCE) return;
+      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;
       }
-      const found = session.page.imageAt(x, y, 0);
+      // A tap on empty paper: the second within DOUBLE_TAP_MS undoes.
-      if (found && session.removeImage(found.index)) log(`Image ${found.index + 1} removed`);
+      const now = performance.now();
+      if (now - lastEmptyTap <= DOUBLE_TAP_MS) {
+        lastEmptyTap = 0;
+        undo();
+      } else {
+        lastEmptyTap = now;
+      }
     },
     previewRect: (x0, y0, x1, y1) => {
       if (Math.abs(x1 - x0) >= TAP_DISTANCE || Math.abs(y1 - y0) >= TAP_DISTANCE) {
@@ -1170,7 +1248,12 @@ function press(group, chosen) {
 function buildToolbar(options) {
   for (const button of document.querySelectorAll('[data-tool]')) {
     button.addEventListener('click', () => {
-      press(/** @type {Element} */ (button.parentElement), button);
+      // The tool buttons stand in two groups since 2026-10-11 (Pen and
+      // Text before the colours they use, Eraser and Hide 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));
+      }
       const chosen = button.getAttribute('data-tool');
       options.setTool(chosen === 'eraser' ? 'eraser' : chosen === 'text' ? 'text' : chosen === 'hide' ? 'hide' : 'pen');
     });

client/js/pen_input.js

@@ -29,7 +29,7 @@ const NOMINAL_PRESSURE = 0.4;
  *     Erases one's own strokes and text blocks along the movement and says how many.
  * @property {(x: number, y: number, reach: number) => void} [eraseEnded]
  *     The eraser went down at (x, y) and has come up again, having moved
- *     no farther than `reach` from there.
+ *     no farther than `reach` from there; Infinity if the press was cancelled.
  * @property {(x0: number, y0: number, x1: number, y1: number) => void} placeRect
  *     With the text or Hide tool, the pointer went down at (x0, y0) and up
  *     at (x1, y1): a tap, or a rectangle dragged open.
@@ -292,7 +292,8 @@ export function attachPenInput(surface, target) {
       if (event.type === 'pointerup') target.placeRect(textStartX, textStartY, x, y);
     } else if (erasing) {
       eraseFromPress();
-      if (event.type === 'pointerup') target.eraseEnded?.(eraseStartX, eraseStartY, eraseReach);
+      // A cancelled press ends the gesture too, with no tap in it.
+      target.eraseEnded?.(eraseStartX, eraseStartY, event.type === 'pointerup' ? eraseReach : Infinity);
     } else {
       target.endStroke();
     }

client/js/selection_guard.js

@@ -46,7 +46,7 @@ function describe(node) {
 }
 
 /** Whether the selection belongs to a focused text field. */
-function inField() {
+export function inField() {
   const active = document.activeElement;
   return active instanceof HTMLInputElement || active instanceof HTMLTextAreaElement ||
          (active instanceof HTMLElement && active.isContentEditable);

client/js/session.js

@@ -23,6 +23,20 @@ import { RelayConnection } from './relay_connection.js';
 /** @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
@@ -55,6 +69,8 @@ import { RelayConnection } from './relay_connection.js';
  *     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.
  */
 
 /**
@@ -72,6 +88,9 @@ import { RelayConnection } from './relay_connection.js';
 /** 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;
 
@@ -106,6 +125,19 @@ export class Session {
     /** @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;
@@ -296,18 +328,41 @@ export class Session {
   addHide(rect) {
     const page = this.page;
     if (!this.mayMakePages() || page.number === 0 || page.hides.length >= MAX_PAGE_HIDES) return false;
-    page.hides.push({
+    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
@@ -332,12 +387,14 @@ export class Session {
    */
   removeHide(index) {
     const page = this.page;
-    if (!this.mayMakePages() || !page.hides[index]) return false;
+    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;
   }
 
@@ -550,10 +607,44 @@ export class Session {
     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);
@@ -727,6 +818,11 @@ export class Session {
     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);
@@ -811,7 +907,11 @@ export class Session {
     this.send(0, { type: 'stroke_points', id: pending.id, at: pending.at, points: pending.points });
   }
 
-  endStroke() {
+  /**
+   * @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;
@@ -819,18 +919,125 @@ export class Session {
     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.
 
   /**
@@ -1080,6 +1287,22 @@ export class Session {
         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;
@@ -1245,3 +1468,27 @@ export class Session {
     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 };
+}

client/js/text_editor.js

@@ -30,6 +30,8 @@ export class TextEditor {
    * @param {object} target
    * @param {(block: TextBlock) => void} target.setText   The block changed.
    * @param {(id: string) => void} target.removeText      The block was left empty.
+   * @param {(id: string) => void} [target.beginEdit]     Editing of the block begins: one gesture until endEdit.
+   * @param {(id: string) => void} [target.endEdit]       The editing ended.
    * @param {(clientX: number, clientY: number) => boolean} [target.beginHold]
    *     The pen rested on the field for HOLD_MS: a hold, as on the page
    *     (Andy, 2026-10-09: the box being typed in can be moved without
@@ -185,6 +187,7 @@ export class TextEditor {
    */
   edit(block) {
     this.stop();
+    this.target.beginEdit?.(block.id);
     this.block = block;
     this.renderer.editingText = block.id;
     this.field.value = block.text;
@@ -207,6 +210,7 @@ export class TextEditor {
     this.field.value = '';
     if (block.text.trim() === '') this.target.removeText(block.id);
     else this.target.setText(block);
+    this.target.endEdit?.(block.id);
   }
 
   /**

client/kokkoja.css

@@ -102,13 +102,35 @@ h2 {
   background: var(--panel);
 }
 
+/* Three groups (Andy, 2026-10-11): the drawing tools with their settings
+   and Undo on the left; the page and the view in the middle, centred
+   between the other two; the session's buttons flush right.  The
+   drawing group keeps the width of its widest state (main.js sets its
+   min-width by measuring), so that the middle group stays put when the
+   tool changes. */
+.drawing, .settings { display: flex; align-items: center; gap: 0.5rem 0.55rem; }
+/* Twice the gap before Eraser, Hide and Undo, so that with the pen the
+   widest line and the Eraser read as two groups (Andy, 2026-10-11). */
+.drawing .after_settings { margin-left: 0.55rem; }
+.toolbar .page_view { margin-left: auto; margin-right: auto; }
+
 /* When the window is too narrow for one row, the tools are centred
    instead of standing at the two ends (Andy, 2026-10-09; main.js sets
    the class by measuring). */
 .toolbar.wrapped { justify-content: center; }
+.toolbar.wrapped .page_view { margin-left: 0; margin-right: 0; }
 .toolbar.wrapped .toolbar_right { margin-left: 0; justify-content: center; }
 
-.toolbar .notebook_pick { width: 9rem; }
+/* 9rem wide until 2026-10-11: room for Undo.  The buttons' height, else
+   this select, 36 px tall by its own padding, sets the row's height and
+   the buttons' trimming gains nothing (measured 2026-10-11). */
+.toolbar .notebook_pick, .toolbar #session_notebook_name {
+  width: 7.2rem;
+  height: calc(2rem - 2px);
+  padding-top: 0;
+  padding-bottom: 0;
+  box-sizing: border-box;
+}
 .toolbar .toggle { display: flex; align-items: center; gap: 0.3rem; cursor: pointer; }
 .toolbar .toggle input { margin: 0; }
 
@@ -125,14 +147,16 @@ h2 {
 
 /* The same height as the buttons, so the toolbar does not change height;
    narrow enough that a host's toolbar fits a 1920-pixel screen in one
-   row (measured 2026-10-09: about 1750 pixels with these settings). */
+   row (measured 2026-10-09: about 1750 pixels with these settings).
+   Since 2026-10-11 two pixels less in height and on each side than
+   2rem and 0.6rem, for room (Andy, in two steps of a pixel). */
 .toolbar button:not(.swatch), #text_size, #text_font {
   box-sizing: border-box;
-  height: 2rem;
+  height: calc(2rem - 2px);
   padding-top: 0;
   padding-bottom: 0;
 }
-.toolbar button:not(.swatch) { padding-left: 0.6rem; padding-right: 0.6rem; }
+.toolbar button:not(.swatch) { padding-left: calc(0.6rem - 2px); padding-right: calc(0.6rem - 2px); }
 
 #text_size, #text_font {
   margin: 0;
@@ -196,7 +220,7 @@ h2 {
   flex-wrap: wrap;
   align-items: center;
   gap: 0.5rem 0.55rem;
-  margin-left: auto;
+  margin-left: 0;   /* auto until 2026-10-11; the middle group's margins push it right now. */
   min-width: 0;
 }
 

docs/guest.md

@@ -48,10 +48,11 @@ place in it waits for the host to return.
 
 ## The screen
 
-Across the top is the toolbar, one row holding everything: the tools
+Across the top is the toolbar, one row holding everything in three
-and their settings, the view, the host's page buttons, your notebook,
+groups: on the left the tools, their settings and **Undo**; in the
-and **Session**, which opens a window with the participants and the
+middle the page and the view; on the right the host's page buttons,
-invitation.  Below it the page has the whole width of the window.  Every button
+your notebook, and **Session**, which opens a window with the
+participants and the invitation.  Below it the page has the whole width of the window.  Every button
 explains itself: rest the pointer on it for two seconds and its
 explanation appears beneath it.
 
@@ -117,7 +118,24 @@ and to the next ones.
 **Eraser** removes your own writing.  Move it across a stroke and the
 whole stroke goes; across a text block and the block goes.  It removes
 nothing of anyone else's, and nobody else's eraser can touch yours.  An
-erased stroke is gone from the page for everyone.
+erased stroke is gone from the page for everyone, until you undo the
+erasure.
+
+### Undo
+
+**Undo**, the last button of the drawing tools, takes back your latest mark: a
+stroke, what one movement of the eraser removed, a text block as you
+last typed or moved it, a Hide rectangle made, moved or removed.  Press
+it again for the mark before that, and so on back to the beginning of
+the page.  Ctrl+Z (Command+Z on a Mac) does the same, and so do two
+quick taps of the Eraser on empty paper, away from anything the eraser
+would remove; a tap that erases something is never half of such a
+double tap.  Undo acts only on your own marks, since nobody can change
+anyone else's, and only on marks made on this page since you opened it:
+after a reload of the tab, or on a page loaded from a notebook, there is
+nothing to undo although the marks are there.  The button is grey when
+there is nothing to undo.  An image cannot be undone; put it back with
+**Image** if a tap of the eraser removed it by mistake.
 
 ### The view
 

docs/host.md

@@ -195,7 +195,8 @@ anyone included, is covered in white, marked by blue corners that tell
 the students something is there.  Use it to cover a solution until the
 student has tried, or a part of the exercise for later.  Rest the pen on
 it to move it, near a corner to resize it; a tap of the eraser removes
-it and shows what was covered.  Nothing under a Hide is lost, and a
+it and shows what was covered, and **Undo** takes back any of these as
+it does your writing.  Nothing under a Hide is lost, and a
 student cannot uncover it by hiding your writing on their screen: a Hide
 belongs to the page, not to your writing.  A Hide is white in an
 exported picture and in the page file as other programs see it.

docs/implementation.md

@@ -151,6 +151,17 @@ layer from that participant.  A client that reconnects after a dropped
 connection sends its whole layer again, so that nothing drawn meanwhile
 is lost.
 
+Undo is personal, since nobody can change another's layer, and is a
+stack of one's own gestures kept in the browser tab for the page being
+written: a stroke; the strokes and text blocks one movement of the
+eraser removed, kept whole; a text block as it was before an editing or
+a move; a Hide rectangle as it was before it was made, moved or removed.
+Undoing sends the ordinary messages, except that erased strokes come
+back by a `stroke_restore` message carrying them whole.  Images are not
+in the stack, because their data is not kept.  The stack is cleared when
+the page changes and is not in the page file, so a reloaded tab starts
+with nothing to undo.
+
 ## The page file
 
 A notebook is a directory.  The page being written is `Current.svg` in
@@ -264,7 +275,10 @@ by which the page grows (`page_model.js`), the tooltip delay
 slack, `pen_input.js`), the reach of an image's corner (24 pixels,
 `renderer.js`), the smallest an image can be scaled to (1 cm,
 `page_model.js`), how often a dragged image's place is sent (40 ms,
-`session.js`) and the points of a stroke (25 ms, `session.js`), the colour and width of a text rectangle's frame
+`session.js`) and the points of a stroke (25 ms, `session.js`), the
+interval of the eraser's double tap (400 ms, `DOUBLE_TAP_MS` in
+`main.js`) and how many gestures can be undone (100, `UNDO_LIMIT` in
+`session.js`), the colour and width of a text rectangle's frame
 (`page_file.js`) and whether the frame is shown whole or as corners on
 screen (`TEXT_FRAME_STYLE` in `renderer.js`; the file and the export
 always hold the whole frame).

tests/test_end_to_end.py

@@ -368,6 +368,37 @@ ERASE_THROUGH = """
     page.setPointerCapture = capture;
 """
 
+# Taps the eraser `count` times at (x, y) in page units, `gap` ms apart,
+# and calls the last argument when done.
+ERASER_TAPS = """
+    const [x, y, count, gap, done] = arguments;
+    const page = document.getElementById('page');
+    const box = page.getBoundingClientRect();
+    const extent = window.kokkoja.session.page;
+    const cx = box.left + (x - extent.left) / extent.width * box.width;
+    const cy = box.top + (y - extent.top) / extent.height * box.height;
+    const capture = page.setPointerCapture;
+    page.setPointerCapture = () => {};
+    const fire = (type, buttons) => page.dispatchEvent(
+      new PointerEvent(type, {pointerId: 7, pointerType: 'pen', isPrimary: true,
+        clientX: cx, clientY: cy, pressure: 0.5,
+        button: type === 'pointermove' ? -1 : 0, buttons, bubbles: true, cancelable: true}));
+    document.querySelector('[data-tool=eraser]').click();
+    let left = count;
+    const tap = () => {
+      fire('pointerdown', 1);
+      fire('pointerup', 0);
+      if (--left > 0) {
+        setTimeout(tap, gap);
+      } else {
+        document.querySelector('[data-tool=pen]').click();
+        page.setPointerCapture = capture;
+        done();
+      }
+    };
+    tap();
+"""
+
 
 def free_port():
     with socket.socket() as listener:
@@ -467,6 +498,12 @@ class Firefox:
             "actions": [{"type": "pointerMove", "duration": 0, "origin": "viewport",
                          "x": round(box[0]), "y": round(box[1])}]}]})
 
+    def chord(self, *keys):
+        """Press keys together and release them, as a keyboard shortcut."""
+        actions = ([{"type": "keyDown", "value": key} for key in keys] +
+                   [{"type": "keyUp", "value": key} for key in reversed(keys)])
+        self.in_session("POST", "/actions", {"actions": [{"type": "key", "id": "keyboard", "actions": actions}]})
+
     def set_window(self, width, height):
         self.in_session("POST", "/window/rect", {"width": width, "height": height})
 
@@ -914,6 +951,136 @@ class EndToEndTest(unittest.TestCase):
         firefox.switch_to(host_tab)
         firefox.go("about:blank")
 
+    def test_undo(self):
+        """Andy, 2026-10-11: one's latest gesture is taken back, then the
+        one before, by the Undo button, Ctrl+Z, or two quick taps of the
+        eraser on empty paper; a stroke, an erasure, text, a Hide."""
+        firefox = self.firefox
+        host_tab = firefox.current_tab()
+        firefox.go(self.address("Late"))
+        self.enter_name("Writer", "start_session")
+        invitation = firefox.run("return document.getElementById('invitation').value;")
+        guest_tab = firefox.new_tab()
+        firefox.go(self.address("Student", join=invitation))
+        self.enter_name("Watcher", "join_session")
+        self.wait_until(lambda s: s["page"]["number"] == 1 and len(s["participants"]) == 2, "the watcher")
+        firefox.switch_to(host_tab)
+        undo_disabled = lambda: firefox.run("return document.getElementById('undo').disabled;")
+        undo = lambda: firefox.run("document.getElementById('undo').click();")
+        self.assertTrue(undo_disabled(), "nothing to undo on a new page")
+
+        # Two strokes; Undo takes back the second, for everyone.
+        firefox.drag("#page", 200, 300, 500, 320)
+        firefox.drag("#page", 200, 400, 500, 420)
+        two = self.wait_until(lambda s: len(s["layers"]["Writer"]) == 2 and s["layers"]["Writer"][-1]["done"],
+                              "two strokes")
+        self.assertFalse(undo_disabled())
+        undo()
+        one = self.wait_until(lambda s: len(s["layers"]["Writer"]) == 1, "one stroke after the undo")
+        self.assertEqual(one["layers"]["Writer"][0]["id"], two["layers"]["Writer"][0]["id"])
+        firefox.switch_to(guest_tab)
+        self.wait_until(lambda s: len(s["layers"].get("Writer", [])) == 1, "the undo to reach the watcher")
+        firefox.switch_to(host_tab)
+
+        # An erasure is one gesture: undone, the stroke is back whole,
+        # with its id, on every screen.
+        stroke = one["layers"]["Writer"][0]
+        page_box = firefox.run("const b = document.getElementById('page').getBoundingClientRect(); return [b.width, b.height];")
+        firefox.run(ERASE_THROUGH, 350 / page_box[0], 310 / page_box[1])
+        self.wait_until(lambda s: len(s["layers"]["Writer"]) == 0, "the stroke erased")
+        self.assertFalse(undo_disabled())
+        undo()
+        back = self.wait_until(lambda s: len(s["layers"]["Writer"]) == 1, "the stroke back")
+        self.assertEqual(back["layers"]["Writer"][0], stroke)
+        firefox.switch_to(guest_tab)
+        watcher = self.wait_until(lambda s: len(s["layers"].get("Writer", [])) == 1 and s["layers"]["Writer"][0]["done"],
+                                  "the restored stroke to reach the watcher")
+        self.assertEqual(watcher["layers"]["Writer"][0], stroke)
+        firefox.switch_to(host_tab)
+
+        # Ctrl+Z undoes the stroke itself, the gesture before the erasure;
+        # then there is nothing left to undo.
+        firefox.chord("\ue009", "z")
+        self.wait_until(lambda s: len(s["layers"]["Writer"]) == 0, "the stroke gone by Ctrl+Z")
+        self.assertTrue(undo_disabled())
+
+        # Two quick taps of the eraser on empty paper undo; one tap, or two
+        # far apart, do nothing; a tap that erases is not half of a double
+        # tap, so the stroke it erased stays erased.
+        origin = self.state()["origin"]
+        vscale = firefox.run("return window.kokkoja.renderer.cssPixelsPerUnit;")
+        units = lambda cx, cy: (origin[0] + cx / vscale, origin[1] + cy / vscale)
+        empty = units(700, 150)
+        firefox.drag("#page", 200, 400, 500, 420)
+        self.wait_until(lambda s: len(s["layers"]["Writer"]) == 1 and s["layers"]["Writer"][0]["done"], "a stroke")
+        firefox.run_until_done(ERASER_TAPS, *empty, 1, 0)
+        time.sleep(0.6)
+        firefox.run_until_done(ERASER_TAPS, *empty, 1, 0)
+        time.sleep(0.2)
+        self.assertEqual(len(self.state()["layers"]["Writer"]), 1, "taps far apart undo nothing")
+        firefox.run_until_done(ERASER_TAPS, *empty, 2, 50)
+        self.wait_until(lambda s: len(s["layers"]["Writer"]) == 0, "the stroke undone by a double tap")
+        firefox.drag("#page", 200, 400, 500, 420)
+        self.wait_until(lambda s: len(s["layers"]["Writer"]) == 1 and s["layers"]["Writer"][0]["done"], "a stroke again")
+        firefox.run_until_done(ERASER_TAPS, *units(350, 410), 1, 0)
+        self.wait_until(lambda s: len(s["layers"]["Writer"]) == 0, "the stroke erased by a tap")
+        firefox.run_until_done(ERASER_TAPS, *empty, 1, 0)
+        time.sleep(0.2)
+        self.assertEqual(len(self.state()["layers"]["Writer"]), 0, "an erasing tap and an empty one are no double tap")
+        firefox.run_until_done(ERASER_TAPS, *empty, 2, 50)
+        self.wait_until(lambda s: len(s["layers"]["Writer"]) == 1, "the erasure undone by a double tap")
+
+        # Text: typing a block is one gesture, changing it another; undo
+        # restores the text as it was before the change, then removes the
+        # block.
+        firefox.run("document.querySelector('[data-tool=text]').click();")
+        firefox.run(TEXT_GESTURE, 0.6, 0.6, 0.6, 0.6)
+        firefox.type_into(".text_field", "Hallo")
+        firefox.run("document.querySelector('[data-tool=pen]').click();")
+        typed = self.wait_until(lambda s: len(s["texts"]["Writer"]) == 1 and s["texts"]["Writer"][0]["text"] == "Hallo",
+                                "the text block")
+        block = typed["texts"]["Writer"][0]
+        state = self.state()
+        fx = (block["x"] + 10 - state["origin"][0]) / state["page"]["width"]
+        fy = (block["y"] + 10 - state["origin"][1]) / state["page"]["height"]
+        firefox.run("document.querySelector('[data-tool=text]').click();")
+        firefox.run(TEXT_GESTURE, fx, fy, fx, fy)
+        firefox.type_into(".text_field", " Welt")
+        firefox.run("document.querySelector('[data-tool=pen]').click();")
+        self.wait_until(lambda s: s["texts"]["Writer"][0]["text"] == "Hallo Welt", "the changed text")
+        undo()
+        self.wait_until(lambda s: s["texts"]["Writer"][0]["text"] == "Hallo", "the text as it was")
+        undo()
+        self.wait_until(lambda s: len(s["texts"]["Writer"]) == 0, "the block gone")
+        self.assertEqual(len(self.state()["layers"]["Writer"]), 1)
+
+        # A Hide rectangle made, and one removed by the eraser, are undone.
+        firefox.run("document.querySelector('[data-tool=hide]').click();")
+        firefox.run(TEXT_GESTURE, 0.05, 0.2, 0.35, 0.4)
+        made = self.wait_until(lambda s: len(s["hides"]) == 1, "the Hide rectangle")
+        undo()
+        self.wait_until(lambda s: not s["hides"], "the Hide undone")
+        firefox.run(TEXT_GESTURE, 0.05, 0.2, 0.35, 0.4)
+        made = self.wait_until(lambda s: len(s["hides"]) == 1, "the Hide rectangle again")
+        hide = made["hides"][0]
+        firefox.run(ERASER_AT, hide[0] + 50, hide[1] + 50, 5)
+        self.wait_until(lambda s: not s["hides"], "the Hide removed by the eraser")
+        undo()
+        self.wait_until(lambda s: s["hides"] == [hide], "the Hide back where it was")
+        firefox.switch_to(guest_tab)
+        self.wait_until(lambda s: s["hides"] == [hide], "the restored Hide to reach the watcher")
+        firefox.switch_to(host_tab)
+
+        # A new page starts with nothing to undo.
+        firefox.run("document.getElementById('new_page').click();")
+        self.wait_until(lambda s: s["page"]["number"] == 2, "the new page")
+        self.assertTrue(undo_disabled())
+
+        firefox.switch_to(guest_tab)
+        firefox.in_session("DELETE", "/window")
+        firefox.switch_to(host_tab)
+        firefox.go("about:blank")
+
     def test_images_are_moved_and_scaled_by_hand(self):
         """The teacher's requests of 2026-10-08: composition mode, and
         images moved, scaled and brought to the front by a hold of the pen."""