feat: show window instantly with cached theme background

src/main/ipc.js

@@ -16,7 +16,7 @@
 const fs = require('fs');
 const path = require('path');
 const {
-  ipcMain, dialog, shell, BrowserWindow,
+  ipcMain, dialog, shell, BrowserWindow, app,
 } = require('electron');
 const os = require('os');
 const { spawn } = require('child_process');
@@ -492,6 +492,22 @@ function registerIpc({ openCscopeWindow, getInitialFolder, getStartupLogs }) {
   ipcMain.handle('app:getCliFolder', () => (typeof getInitialFolder === 'function' ? (getInitialFolder() || null) : null));
   ipcMain.handle('app:getStartupLogs', () => (typeof getStartupLogs === 'function' ? getStartupLogs() : []));
 
+  // Renderer caches the current theme background here whenever a theme is
+  // applied. The main process reads it on the next cold start (see
+  // startupBackground() in main.js) to paint the window with the right color
+  // immediately. Only valid hex colors are accepted; everything is wrapped so a
+  // bad value or a write failure returns false instead of crashing the app.
+  ipcMain.handle('app:setStartupBg', (_e, color) => {
+    try {
+      if (typeof color !== 'string' || !/^#[0-9a-fA-F]{3,8}$/.test(color)) return false;
+      const cacheFile = path.join(app.getPath('userData'), 'startup.json');
+      fs.writeFileSync(cacheFile, JSON.stringify({ bg: color }));
+      return true;
+    } catch (_err) {
+      return false;
+    }
+  });
+
   // ---- tool updates (ripgrep / fd) ----
   ipcMain.handle('tool:checkUpdate', async (_e, params) => {
     try { return await toolUpdate.checkUpdate(params || {}); } catch (err) { return { ok: false, error: String((err && err.message) || err) }; }

src/main/main.js

@@ -61,6 +61,29 @@ let mainWindow = null;
 // a race with the window's did-finish-load event.
 let initialFolder = null;
 
+// Default theme background (DAYLIGHT `--bg` in src/renderer/js/themes.js). Used
+// as the window backgroundColor when no valid startup cache is available.
+const DEFAULT_STARTUP_BG = '#f4f4f4';
+const HEX_COLOR_RE = /^#[0-9a-fA-F]{3,8}$/;
+
+// Read the cached theme background written by the renderer on the previous run
+// (userData/startup.json -> { "bg": "#rrggbb" }). Showing the window with the
+// right backgroundColor avoids a white flash before the renderer paints, which
+// matters most for dark-theme users. Falls back to the light default theme bg
+// when the cache is missing (first launch) or invalid. Wrapped in try/catch so
+// a missing file or bad JSON never throws during boot.
+function startupBackground() {
+  try {
+    const cacheFile = path.join(app.getPath('userData'), 'startup.json');
+    const raw = fs.readFileSync(cacheFile, 'utf8');
+    const bg = JSON.parse(raw).bg;
+    if (typeof bg === 'string' && HEX_COLOR_RE.test(bg)) return bg;
+  } catch (_e) {
+    /* no cache yet / unreadable / invalid JSON - use the default */
+  }
+  return DEFAULT_STARTUP_BG;
+}
+
 function findIcon() {
   const candidates = [
     // Primary location: LOGO/M2_SCOUT.ico next to the app.
@@ -87,7 +110,12 @@ function createMainWindow() {
     title: `M2_SCOUT v${APP_VERSION}`,
     icon: findIcon(),
     autoHideMenuBar: true,
-    backgroundColor: '#f4f4f4',
+    // Show the window immediately on creation (no `show: false` +
+    // `ready-to-show` wait) so it appears as early as possible; the renderer
+    // content fills in right after. backgroundColor is seeded from the cached
+    // theme bg so dark-theme users don't see a white flash first.
+    show: true,
+    backgroundColor: startupBackground(),
     webPreferences: {
       preload: path.join(__dirname, '..', 'preload', 'preload.js'),
       contextIsolation: true,

src/preload/preload.js

@@ -115,6 +115,9 @@ contextBridge.exposeInMainWorld('m2scout', {
   // app events
   getCliFolder: () => ipcRenderer.invoke('app:getCliFolder'),
   getStartupLogs: () => ipcRenderer.invoke('app:getStartupLogs'),
+  // Cache the current theme background so the next cold start can paint the
+  // window with the correct color immediately (avoids a white flash).
+  setStartupBg: (color) => ipcRenderer.invoke('app:setStartupBg', color),
   onCliFolder: (cb) => {
     const listener = (_e, payload) => cb(payload);
     ipcRenderer.on('app:cliFolder', listener);

src/renderer/js/themes.js

@@ -293,6 +293,13 @@
     for (const k of Object.keys(vars)) root.style.setProperty(k, vars[k]);
     root.setAttribute('data-theme', themeId);
     try { localStorage.setItem('appTheme', themeId); } catch (_e) { /* ignore */ }
+    // Cache the current theme background in the main process so the next cold
+    // start paints the window with the correct color immediately (no flash).
+    try {
+      if (window.m2scout && typeof window.m2scout.setStartupBg === 'function') {
+        window.m2scout.setStartupBg(vars['--bg']);
+      }
+    } catch (_e) { /* ignore */ }
     try {
       window.dispatchEvent(new CustomEvent('m2-theme-changed', { detail: { theme: themeId } }));
     } catch (_e) { /* ignore */ }