feat(desktop): window translucency slider in Appearance settings (#45086)

A see-through-window control (0–100, off by default) that maps to the
native window opacity via setOpacity — the desktop shows through the whole
window, the same effect as the Windows shift-scroll trick. macOS + Windows;
a no-op on Linux (no runtime window opacity).

Renderer owns the value (persisted, nanostore) and mirrors it to the main
process over IPC; main persists it to translucency.json so a cold launch
applies it at window creation before the renderer reports in.

apps/desktop/electron/main.cjs

@@ -387,6 +387,58 @@ function writePersistedThemeSource(mode) {
 
 nativeTheme.themeSource = readPersistedThemeSource()
 
+// Window translucency (see-through window). One lever, 0–100; 0 = off (the
+// default). Mapped to the native window opacity so the desktop shows through
+// the whole window. Persisted so a cold launch applies it at window creation,
+// before the renderer reports its value. macOS + Windows only; `setOpacity` is
+// a no-op on Linux. See store/translucency.
+const TRANSLUCENCY_CONFIG_PATH = path.join(app.getPath('userData'), 'translucency.json')
+
+function clampIntensity(value) {
+  const n = Math.round(Number(value))
+
+  return Number.isFinite(n) ? Math.min(100, Math.max(0, n)) : 0
+}
+
+function readPersistedTranslucency() {
+  try {
+    return clampIntensity(JSON.parse(fs.readFileSync(TRANSLUCENCY_CONFIG_PATH, 'utf8')).intensity)
+  } catch {
+    return 0
+  }
+}
+
+function writePersistedTranslucency(intensity) {
+  try {
+    fs.mkdirSync(path.dirname(TRANSLUCENCY_CONFIG_PATH), { recursive: true })
+    fs.writeFileSync(TRANSLUCENCY_CONFIG_PATH, JSON.stringify({ intensity }, null, 2), 'utf8')
+  } catch (error) {
+    rememberLog(`[translucency] write failed: ${error.message}`)
+  }
+}
+
+let translucencyIntensity = readPersistedTranslucency()
+
+// Map the 0–100 lever to a window opacity. Floor at 0.3 so the most see-through
+// setting is still usable rather than nearly invisible. 0 → fully opaque.
+function windowOpacity() {
+  return 1 - (translucencyIntensity / 100) * 0.7
+}
+
+// Re-apply translucency to a live window (runtime toggle, no recreation).
+// `setOpacity` is a no-op on Linux, which is fine — it just stays opaque there.
+function applyWindowTranslucency(win) {
+  if (!win || win.isDestroyed() || typeof win.setOpacity !== 'function') {
+    return
+  }
+
+  try {
+    win.setOpacity(windowOpacity())
+  } catch (error) {
+    rememberLog(`[translucency] apply failed: ${error.message}`)
+  }
+}
+
 function isHexColor(value) {
   return typeof value === 'string' && /^#[0-9a-f]{6}$/i.test(value)
 }
@@ -5028,6 +5080,7 @@ function createSessionWindow(sessionId, { watch = false } = {}) {
       titleBarOverlay: getTitleBarOverlayOptions(),
       trafficLightPosition: IS_MAC ? WINDOW_BUTTON_POSITION : undefined,
       vibrancy: IS_MAC ? 'sidebar' : undefined,
+      opacity: windowOpacity(),
       icon,
       // Don't show until the renderer's first themed paint is ready. macOS
       // `vibrancy` ignores `backgroundColor` and paints a translucent OS
@@ -5092,6 +5145,7 @@ function createWindow() {
     titleBarOverlay: getTitleBarOverlayOptions(),
     trafficLightPosition: IS_MAC ? WINDOW_BUTTON_POSITION : undefined,
     vibrancy: IS_MAC ? 'sidebar' : undefined,
+    opacity: windowOpacity(),
     icon,
     // Hidden until the first themed paint so macOS `vibrancy` (which ignores
     // `backgroundColor` and follows the OS appearance) can't flash a light
@@ -5673,6 +5727,23 @@ ipcMain.on('hermes:native-theme', (_event, mode) => {
   }
 })
 
+// See-through window translucency. Persist + re-apply opacity to every open
+// window at runtime (no recreation, so caching/sessions are untouched).
+ipcMain.on('hermes:translucency', (_event, payload) => {
+  const next = clampIntensity(payload && payload.intensity)
+
+  if (next === translucencyIntensity) {
+    return
+  }
+
+  translucencyIntensity = next
+  writePersistedTranslucency(next)
+
+  for (const win of BrowserWindow.getAllWindows()) {
+    applyWindowTranslucency(win)
+  }
+})
+
 ipcMain.handle('hermes:openExternal', (_event, url) => {
   if (!openExternalUrl(url)) {
     throw new Error('Invalid external URL')

apps/desktop/electron/preload.cjs

@@ -40,6 +40,7 @@ contextBridge.exposeInMainWorld('hermesDesktop', {
   stopPreviewFileWatch: id => ipcRenderer.invoke('hermes:stopPreviewFileWatch', id),
   setTitleBarTheme: payload => ipcRenderer.send('hermes:titlebar-theme', payload),
   setNativeTheme: mode => ipcRenderer.send('hermes:native-theme', mode),
+  setTranslucency: payload => ipcRenderer.send('hermes:translucency', payload),
   setPreviewShortcutActive: active => ipcRenderer.send('hermes:previewShortcutActive', Boolean(active)),
   openExternal: url => ipcRenderer.invoke('hermes:openExternal', url),
   fetchLinkTitle: url => ipcRenderer.invoke('hermes:fetchLinkTitle', url),