manage screen share preview lifecycle

public/app.js

@@ -869,9 +869,26 @@ function onRemoteTrack(peer, ev) {
     track.addEventListener('ended', () => {
       if (peer.screenStreams.get(stream.id) === stream) {
         peer.screenStreams.delete(stream.id);
+        if (!peer.screenStreams.size) {
+          peer.state = { ...peer.state, sharing: false };
+          captureRumIssue('Remote screen share ended', 'INFO', {
+            peerId: peer.id,
+            peerName: peer.name,
+            streamId: stream.id,
+          });
+        }
         renderScreens();
+        renderTiles();
       }
     });
+    stream.addEventListener('inactive', () => {
+      if (peer.screenStreams.get(stream.id) === stream) {
+        peer.screenStreams.delete(stream.id);
+        peer.state = { ...peer.state, sharing: false };
+        renderScreens();
+        renderTiles();
+      }
+    }, { once: true });
     renderScreens();
     if (isNewScreenShare) playRetroCue('share');
     console.log('remote screen share from', peer.name);
@@ -993,6 +1010,10 @@ async function startScreenShare() {
     videoTrack.addEventListener('ended', () => stopScreenShare(stream));
   }
   renderScreens();
+  // Open a desktop preview automatically; mobile keeps the in-page preview.
+  if (window.matchMedia('(min-width: 700px)').matches) {
+    popOutShare({ key: `me:${stream.id}`, name: `${S.name} (you)`, stream, local: true });
+  }
   renderTiles();
   for (const btn of shareButtons) btn.classList.add('active');
   toast('You are sharing your screen', 'ok');