fix(electron): show the main window once its document has loaded; enforce J1 IPC and mutation counters (#1828)

* fix(electron): show the main window once its document has loaded; enforce J1 IPC and mutation counters

Re-lands #1788, which merged into #1782's branch after #1782 had already
reached master, so none of it is on master.

The hidden main window was shown on ready-to-show only. On Linux under
X11, when the startup scripts run before the window's first frame, the
next frame comes about a second later: nothing is on screen and the
splash's requestAnimationFrame waits, so J1's first card came ~940 ms
after load instead of ~480 ms in most runner launches (18 bridge calls /
1,031-1,033 DOM mutations instead of 15 / 558).

The window is now shown at ready-to-show or the main frame's
did-finish-load, whichever comes first, with the splash colour as its
background so showing before the first paint does not flash. The journey
gate keeps the app's did-finish-load listeners away from its about:blank
detour, as it already does for ready-to-show.

Three dispatched runs on this branch (37192092882, 37192097790,
37192103151) read 15 calls and 558 mutations in all 18 iterations,
stable: true. Both become baselines (slack 0), and the Performance
journeys job checks them with check-journey-ratchet.mjs --only.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

* chore(perf): record the evidence PR of the J1 runtime baselines

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>
Co-authored-by: 4gray <fourgray@proton.me>

apps/electron-backend/src/app/app.ts

@@ -15,6 +15,10 @@ import {
     trace,
     traceStartupPhase,
 } from './services/debug-trace';
+import {
+    MAIN_WINDOW_BACKGROUND_COLOR,
+    showMainWindowWhenLoaded,
+} from './services/main-window-first-show';
 import { attachMainWindowPerformanceCounters } from './services/performance-counters';
 import {
     STARTUP_WINDOW_MODE,
@@ -512,6 +516,9 @@ export default class App {
             width: width,
             height: height,
             show: false,
+            // The splash colour: the window can be shown before its first
+            // paint (main-window-first-show.ts).
+            backgroundColor: MAIN_WINDOW_BACKGROUND_COLOR,
             webPreferences: getMainWindowWebPreferences(),
             ...savedWindowBounds,
             // Fullscreen is a constructor option: the window is created
@@ -543,15 +550,17 @@ export default class App {
             App.mainWindow.center();
         }
 
-        // if main window is ready to show, close the splash window and show the main window
-        App.mainWindow.once('ready-to-show', () => {
+        // Shown at ready-to-show or did-finish-load, whichever comes first
+        // (see main-window-first-show.ts).
+        const mainWindow = App.mainWindow;
+        showMainWindowWhenLoaded(mainWindow, () => {
             // maximize() on a hidden window shows it (Electron docs), so it
-            // has to wait for ready-to-show like show() does — any earlier
-            // and a blank window flashes before the renderer paints.
+            // waits for the document like show() does — any earlier and a
+            // blank window flashes before the splash is there.
             if (startupWindowMode === 'maximized') {
-                App.mainWindow.maximize();
+                mainWindow.maximize();
             }
-            App.mainWindow.show();
+            mainWindow.show();
             // macOS ignores the constructor's `fullscreen` while the window
             // is hidden — an NSWindow can only toggle fullscreen once it is
             // on screen — so the request is repeated after show() wherever
@@ -562,9 +571,9 @@ export default class App {
             // asking for it again.
             if (
                 startupWindowMode === 'fullscreen' &&
-                !App.mainWindow.isFullScreen()
+                !mainWindow.isFullScreen()
             ) {
-                requestFullScreen(App.mainWindow, true);
+                requestFullScreen(mainWindow, true);
             }
         });