Expand replaces Widen: fill the window in both directions

From the development repository at 3f1da52.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

client/js/session.js

@@ -479,28 +479,37 @@ export class Session {
   }
 
   /**
-   * Make the sheet wider, for more room to write (Andy, 2026-10-08:
-   * "Widen" enlarges the page rather than zooming).  Everyone's page
-   * widens; 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.
-   * @returns {boolean}  Whether the page widened.
+   * 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.
    */
-  widenPage(width) {
+  expandPage(width, height) {
     const page = this.page;
-    const added = Math.round(width) - page.width;
-    if (!this.mayMakePages() || page.number === 0 || added <= 0) return false;
-    // Equally on both sides 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 page's centre half a unit, a fraction
-    // of a pixel that showed as a one-pixel move on Olion (2026-10-09).
-    const half = Math.floor(added / 2);
-    if (half === 0) return false;
-    page.baseLeft = page.left - half;
-    page.baseWidth = page.width + 2 * half;
+    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);
@@ -510,7 +519,7 @@ export class Session {
   /**
    * 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 "Widen".
+   * page like "Expand".
    * @returns {boolean}  Whether the sheet changed.
    */
   shrinkPage() {