fix: restore setDisplayMediaRequestHandler with useSystemPicker

Electron requires setDisplayMediaRequestHandler for getDisplayMedia() to
work — removing it broke screen sharing entirely. Restored the handler
with useSystemPicker: true, which on macOS 15+ uses the native system
picker (honoring restrictOwnAudio), while Windows/Linux fall back to
the custom picker with the shareAudio toggle for echo control.

packages/desktop/src/main.ts

@@ -9,6 +9,7 @@ import {
   shell,
   screen,
   session,
+  desktopCapturer,
 } from 'electron';
 import path from 'path';
 import fs from 'fs';
@@ -430,6 +431,11 @@ function registerIpcHandlers(): void {
     }
   });
 
+  // Screen share picker coordination (used by setDisplayMediaRequestHandler)
+  ipcMain.on('screen-share-selected', (_event, _sourceId: string | null, _shareAudio?: boolean) => {
+    // Handled via ipcMain.once in the display media handler — this is just
+    // a safety net to prevent unhandled-message warnings
+  });
 }
 
 // ─── Auto-Update ────────────────────────────────────────────────────────────
@@ -591,10 +597,68 @@ if (!gotTheLock) {
     await session.defaultSession.clearStorageData({ storages: ['serviceworkers'] });
     await session.defaultSession.clearCache();
 
-    // Screen sharing: let Chromium's native getDisplayMedia() pipeline handle
-    // source selection and audio capture. This ensures restrictOwnAudio is
-    // applied natively, preventing the app's own voice playback from being
-    // captured in the system audio loopback.
+    // Intercept getDisplayMedia() — show custom picker in renderer.
+    // useSystemPicker: on macOS 15+, the native system picker runs instead of
+    // our handler, allowing Chromium's restrictOwnAudio constraint to work
+    // natively (no audio feedback). On Windows/Linux the handler is called.
+    session.defaultSession.setDisplayMediaRequestHandler(async (_request, callback) => {
+      console.log('[Main:ScreenShare] Handler invoked');
+      try {
+        const sources = await desktopCapturer.getSources({
+          types: ['screen', 'window'],
+          thumbnailSize: { width: 320, height: 180 },
+          fetchWindowIcons: true,
+        });
+        console.log('[Main:ScreenShare] Got', sources.length, 'sources');
+
+        if (sources.length === 0) {
+          console.warn('[Main:ScreenShare] No sources — Screen Recording permission may not be granted');
+          // @ts-ignore — Electron throws if we pass {} when video was requested; pass nothing to deny
+          callback();
+          return;
+        }
+
+        const serialized = sources.map((source) => ({
+          id: source.id,
+          name: source.name,
+          thumbnailDataUrl: source.thumbnail.toDataURL(),
+          appIconDataUrl: source.appIcon && !source.appIcon.isEmpty()
+            ? source.appIcon.toDataURL() : null,
+          isScreen: source.id.startsWith('screen:'),
+        }));
+
+        // Send sources to renderer, wait for user selection
+        mainWindow?.webContents.send('screen-share-sources', serialized);
+
+        const { sourceId, shareAudio } = await new Promise<{ sourceId: string | null; shareAudio: boolean }>((resolve) => {
+          ipcMain.once('screen-share-selected', (_event, id: string | null, wantAudio?: boolean) => {
+            resolve({ sourceId: id, shareAudio: wantAudio ?? true });
+          });
+        });
+        console.log('[Main:ScreenShare] User selected:', sourceId, 'audio:', shareAudio);
+
+        if (!sourceId) {
+          // @ts-ignore — deny the request without crashing
+          callback();
+          return;
+        }
+
+        const selected = sources.find((s) => s.id === sourceId);
+        if (!selected) {
+          // @ts-ignore — deny the request without crashing
+          callback();
+          return;
+        }
+
+        // Provide the selected source — Electron creates the MediaStream
+        // System audio loopback: Windows/Linux native, macOS 13+ via ScreenCaptureKit
+        callback({ video: selected, ...(shareAudio ? { audio: 'loopback' } : {}) });
+      } catch (err) {
+        console.error('[Main:ScreenShare] Handler error:', err);
+        // @ts-ignore — deny the request without crashing
+        callback();
+      }
+    }, { useSystemPicker: true });
 
     registerIpcHandlers();
     createWindow();

packages/desktop/src/preload.ts

@@ -50,6 +50,14 @@ contextBridge.exposeInMainWorld('backspace', {
     ipcRenderer.on('deep-link', (_event, url) => callback(url));
   },
 
+  // Screen share picker coordination
+  onScreenShareSources: (callback: (sources: unknown[]) => void) => {
+    ipcRenderer.on('screen-share-sources', (_event, sources) => callback(sources));
+  },
+  selectScreenSource: (sourceId: string | null, shareAudio?: boolean) => {
+    ipcRenderer.send('screen-share-selected', sourceId, shareAudio ?? true);
+  },
+
   // Instance URL management
   getInstanceUrl: () => ipcRenderer.invoke('get-instance-url'),
   setInstanceUrl: (url: string) => ipcRenderer.invoke('set-instance-url', url),