fix(desktop): keep UI scale across in-page route navigation

Desktop is a HashRouter over one file:// document, so every route is a
distinct URL to Chromium's per-URL zoom store. A route the user never
zoomed on has no record at all and resolves to the host default (100%) —
that is every fresh session and every never-visited settings tab.

In-page navigation fires neither did-finish-load nor any window event,
so nothing re-asserted the persisted level. The window dropped to 100%
while the Appearance control kept reading the chosen scale, because the
renderer only learns of zoom changes through 'hermes:zoom:changed',
which never fired. Touching the setting sent a fresh apply, which is
why it appeared to fix itself.

Re-assert the persisted level on main-frame did-navigate-in-page.
Verified on real Electron 40.10.2 / Chromium 144 (win32): a recordless
hash route reports 100% at the event, so the existing drift-guard sees
the drop and re-applies, and still no-ops when the route's record
already matches.

Fixes #48658
Fixes #38854
Fixes #79863

Co-authored-by: Brooklyn Nicholson <brooklyn.bb.nicholson@gmail.com>

apps/desktop/electron/main.ts

@@ -6557,6 +6557,7 @@ function installPreviewShortcut(window) {
 import {
   applyZoomLevel,
   DEFAULT_ZOOM_LEVEL,
+  installZoomReassertOnNavigation,
   installZoomReassertOnWindowEvents,
   percentToZoomLevel,
   ZOOM_STEP,
@@ -11202,13 +11203,15 @@ function wireCommonWindowHandlers(win, { zoom = true }: { zoom?: boolean } = {})
   if (zoom) {
     installZoomShortcuts(win)
     // Re-apply persisted zoom on show/restore/resize/cross-display move
-    // (Chromium can drop webContents zoom after these window transitions) and
-    // on EVERY full load — not once. The crash-recovery path calls
-    // webContents.reload(), which fires did-finish-load again after a `once`
-    // listener is spent, so zoom was silently lost on renderer crash
-    // recovery and any in-place reload/navigation (#46429).
-    installZoomReassertOnWindowEvents(win, () => restorePersistedZoomLevel(win))
-    win.webContents.on('did-finish-load', () => restorePersistedZoomLevel(win))
+    // (Chromium can drop webContents zoom after these window transitions), on
+    // EVERY full load — not once, since crash recovery reloads and would
+    // outlive a spent `once` listener (#46429) — and after in-page navigation,
+    // where Chromium applies the target hash route's own per-URL zoom record
+    // (see installZoomReassertOnNavigation; #48658, #38854, #79863).
+    const reassertZoom = () => restorePersistedZoomLevel(win)
+
+    installZoomReassertOnWindowEvents(win, reassertZoom)
+    installZoomReassertOnNavigation(win.webContents, reassertZoom)
   }
 
   installContextMenuBridge(win)

apps/desktop/electron/zoom.test.ts

@@ -12,6 +12,7 @@ import {
   applyZoomLevel,
   clampZoomLevel,
   DEFAULT_ZOOM_LEVEL,
+  installZoomReassertOnNavigation,
   installZoomReassertOnWindowEvents,
   isDebouncedReassertEvent,
   percentToZoomLevel,
@@ -292,6 +293,34 @@ test('installZoomReassertOnWindowEvents skips destroyed windows', () => {
   assert.equal(calls, 0)
 })
 
+test('installZoomReassertOnNavigation covers full loads and main-frame in-page routes', () => {
+  const handlers = new Map()
+  let destroyed = false
+  let calls = 0
+
+  const webContents = {
+    isDestroyed: () => destroyed,
+    on(event, listener) {
+      handlers.set(event, listener)
+    }
+  }
+
+  installZoomReassertOnNavigation(webContents, () => {
+    calls += 1
+  })
+
+  assert.deepEqual([...handlers.keys()], ['did-finish-load', 'did-navigate-in-page'])
+
+  handlers.get('did-finish-load')()
+  handlers.get('did-navigate-in-page')({}, 'file:///app/index.html#/new', true)
+  handlers.get('did-navigate-in-page')({}, 'file:///app/frame.html#anchor', false)
+  assert.equal(calls, 2)
+
+  destroyed = true
+  handlers.get('did-navigate-in-page')({}, 'file:///app/index.html#/session/next', true)
+  assert.equal(calls, 2)
+})
+
 // Zoom-wiring contract: chat windows keep global UI zoom while fixed-size
 // helper windows opt out. Tested via the extracted config — no source-text regex.
 test('chat windows opt into zoom', () => {

apps/desktop/electron/zoom.ts

@@ -154,6 +154,45 @@ export function installZoomReassertOnWindowEvents(win, reassert, platform = proc
   }
 }
 
+/**
+ * Chromium persists zoom PER URL, and a hash route is a distinct URL. Desktop
+ * is a HashRouter over one `file://index.html`, so every session/settings route
+ * carries its own `partition.per_host_zoom_levels` record, and an in-page
+ * navigation applies the target route's record over whatever the window is
+ * showing. A route the user never zoomed on has no record at all, so it
+ * resolves to the host default — level 0, i.e. 100%.
+ *
+ * In-page navigation fires neither `did-finish-load` nor any window event, so
+ * nothing re-asserted the persisted level: opening a fresh session dropped the
+ * window to 100% while the Appearance control kept reading the chosen scale (it
+ * only learns of changes through `hermes:zoom:changed`, which never fired —
+ * which is why touching the setting appeared to fix it). #48658, #38854, #79863.
+ *
+ * Verified on real Electron 40.10.2 / Chromium 144 (win32): at
+ * `did-navigate-in-page` the frame already reports the target route's level, so
+ * restorePersistedZoomLevel's drift-guard sees the drop and re-applies — and
+ * still no-ops when the route's record already matches. Subframe hash changes
+ * must not touch the chat window's UI scale.
+ */
+export function installZoomReassertOnNavigation(webContents, reassert) {
+  if (!webContents?.on) {
+    return
+  }
+
+  const reassertIfAlive = () => {
+    if (!webContents.isDestroyed?.()) {
+      reassert()
+    }
+  }
+
+  webContents.on('did-finish-load', reassertIfAlive)
+  webContents.on('did-navigate-in-page', (_event, _url, isMainFrame) => {
+    if (isMainFrame) {
+      reassertIfAlive()
+    }
+  })
+}
+
 /**
  * Zoom-wiring decision per window kind. Chat windows (main + session + the HUD)
  * keep global UI zoom; the pet overlay and the Quick Entry composer opt out