Surface screen share failures instead of silently doing nothing

toggleScreenSharing only had `.catch(logger.error)`, so a getDisplayMedia
request that hangs (element-call-rageshakes#17152: Element Desktop on
Windows, the user pressed the screen share button 14 times in 25 seconds
and the log shows nothing but the toggle lines and livekit-client's
"waiting for pending publication promise timed out") left the user with
a button that does nothing and us with no evidence of why.

Log when a toggle is requested and when it completes or fails, with the
elapsed time, so a hang is visible in the logs. Explicit failures other
than the user cancelling the picker show as a non-modal "Could not start
screen sharing" toast. Nothing is inferred from a toggle taking a long
time: the user may simply be choosing what to share.

src/state/CallViewModel/CallViewModel.ts

@@ -267,6 +267,11 @@ export interface CallViewModel {
    * Whether we are sharing our screen.
    */
   sharingScreen$: Behavior<boolean>;
+  /**
+   * The last error from toggling screen sharing, until dismissed.
+   */
+  screenShareError$: Behavior<Error | null>;
+  dismissScreenShareError: () => void;
 
   // UI interactions
   /**
@@ -1858,6 +1863,8 @@ export function createCallViewModel$(
     reconnecting$: localMembership.reconnecting$,
     livekitRoomItems$,
     connected$: localMembership.connected$,
+    screenShareError$: localMembership.screenShareError$,
+    dismissScreenShareError: localMembership.dismissScreenShareError,
   };
 }
 

src/state/CallViewModel/localMember/LocalMember.test.ts

@@ -41,6 +41,7 @@ import {
   enterRTCSession,
   PublishState,
   TrackState,
+  watchScreenShareToggle,
 } from "./LocalMember";
 import {
   FailToGetOpenIdToken,
@@ -68,6 +69,31 @@ vi.mock("@livekit/components-core", () => ({
     .mockReturnValue(of({ isScreenShareEnabled: false })),
 }));
 
+describe("watchScreenShareToggle", () => {
+  it("reports nothing when the toggle completes", async () => {
+    const onError = vi.fn();
+    watchScreenShareToggle(Promise.resolve(), true, logger, onError);
+    await flushPromises();
+    expect(onError).not.toHaveBeenCalled();
+  });
+
+  it("reports failures other than the user cancelling", async () => {
+    const onError = vi.fn();
+    const e = new Error("NotReadableError");
+    watchScreenShareToggle(Promise.reject(e), true, logger, onError);
+    await flushPromises();
+    expect(onError).toHaveBeenCalledWith(e);
+  });
+
+  it("does not report the user cancelling the picker", async () => {
+    const onError = vi.fn();
+    const cancelled = new DOMException("Permission denied", "NotAllowedError");
+    watchScreenShareToggle(Promise.reject(cancelled), true, logger, onError);
+    await flushPromises();
+    expect(onError).not.toHaveBeenCalled();
+  });
+});
+
 describe("LocalMembership", () => {
   describe("enterRTCSession", () => {
     it("It joins the correct Session", () => {

src/state/CallViewModel/localMember/LocalMember.ts

@@ -196,6 +196,11 @@ export const createLocalMembership$ = ({
    * Callback to toggle screen sharing. If null, screen sharing is not possible.
    */
   toggleScreenSharing: (() => void) | null;
+  /**
+   * The last error from toggling screen sharing, until dismissed.
+   */
+  screenShareError$: Behavior<Error | null>;
+  dismissScreenShareError: () => void;
   // tracks$: Behavior<LocalTrack[]>;
   participant$: Behavior<LocalParticipant | null>;
   connection$: Behavior<Connection | null>;
@@ -706,6 +711,7 @@ export const createLocalMembership$ = ({
     ),
   );
 
+  const screenShareError$ = new BehaviorSubject<Error | null>(null);
   let toggleScreenSharing: (() => void) | null = null;
   if (
     "getDisplayMedia" in (navigator.mediaDevices ?? {}) &&
@@ -777,13 +783,18 @@ export const createLocalMembership$ = ({
       // We also allow screen sharing to be toggled even if the connection
       // is still initializing or publishing tracks, because there's no
       // technical reason to disallow this. LiveKit will publish if it can.
-      participant$.value
-        ?.setScreenShareEnabled(
+      const participant = participant$.value;
+      if (!participant) return;
+      watchScreenShareToggle(
+        participant.setScreenShareEnabled(
           targetScreenshareState,
           screenshareSettings,
           publishOptions,
-        )
-        .catch(logger.error);
+        ),
+        targetScreenshareState,
+        logger,
+        (e) => screenShareError$.next(e),
+      );
     };
   }
 
@@ -802,11 +813,44 @@ export const createLocalMembership$ = ({
     ),
     sharingScreen$,
     toggleScreenSharing,
+    screenShareError$,
+    dismissScreenShareError: () => screenShareError$.next(null),
     connection$: localConnection$,
     internalLoggerRef: logger,
   };
 };
 
+/**
+ * Logs the outcome of a screen share toggle and reports failures.
+ *
+ * getDisplayMedia may legitimately take a long time (the user is choosing
+ * what to share) or never settle at all, so nothing is inferred from silence:
+ * the request and its completion are logged with the elapsed time so that a
+ * hang is visible in the logs, and only an explicit rejection is reported.
+ *
+ * The user cancelling the picker rejects with a NotAllowedError; that is
+ * logged but not reported.
+ */
+export function watchScreenShareToggle(
+  toggle: Promise<unknown>,
+  enable: boolean,
+  logger: Logger,
+  onError: (e: Error) => void,
+): void {
+  const what = `Screen share ${enable ? "start" : "stop"}`;
+  const started = Date.now();
+  const elapsed = (): string => `${Date.now() - started} ms`;
+  logger.info(`${what} requested`);
+  toggle.then(
+    () => logger.info(`${what} completed in ${elapsed()}`),
+    (e: unknown) => {
+      logger.error(`${what} failed after ${elapsed()}:`, e);
+      if (e instanceof DOMException && e.name === "NotAllowedError") return;
+      onError(e instanceof Error ? e : new Error(String(e)));
+    },
+  );
+}
+
 export function observeSharingScreen$(p: Participant): Observable<boolean> {
   return observeParticipantEvents(
     p,