feat(lotus): io.lotus.set_screenshare for the host call bar (cinny #43)

toWidget `io.lotus.set_screenshare { on?: boolean }` (omit to toggle)
starts/stops sharing via the view model instead of the host clicking EC's
hidden button. The host sends it with Capability Delegation
(`delegate: "display-capture"`) where supported, so getDisplayMedia keeps
the user's click on engines that require it. `controls_state` now carries
`screenshareAction: true` so the host only uses it on a fork that has it.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PPmy3tPq869XDW4njjVaKA

src/lotus/lotusControls.ts

@@ -21,6 +21,8 @@ export interface LotusControlsState {
   screensharing: boolean;
   /** Null while EC offers no layout switch (e.g. PiP or a 1:1 layout). */
   layout: LayoutMode | null;
+  /** This fork handles `io.lotus.set_screenshare` (always true). */
+  screenshareAction: true;
 }
 
 /** `{ layout }` payload → a layout mode, or undefined if invalid. Exported for tests. */
@@ -39,13 +41,28 @@ export function parseSettingsPayload(data: unknown, current: boolean): boolean {
   return !current;
 }
 
+/**
+ * `{ on? }` payload + current sharing state → whether to call the toggle.
+ * Exported for tests.
+ */
+export function shouldToggleScreenshare(
+  data: unknown,
+  sharing: boolean,
+): boolean {
+  if (typeof data === "object" && data !== null && "on" in data) {
+    const { on } = data as { on?: unknown };
+    if (typeof on === "boolean") return on !== sharing;
+  }
+  return true;
+}
+
 /**
  * [cinny #43] Widget-API replacements for the host's DOM access to EC's
  * controls: layout switch, settings modal and reactions menu, plus a
  * `controls_state` report (screensharing + layout) so the host no longer reads
- * EC's DOM for them. Screensharing itself stays host-DOM driven for now:
- * `getDisplayMedia` needs the user's click to reach this frame (Capability
- * Delegation), which a plain widget message doesn't carry.
+ * EC's DOM for them. Screenshare start/stop too: `getDisplayMedia` needs the
+ * user's click in this frame, so the host sends `set_screenshare` with
+ * Capability Delegation (Chromium). Elsewhere it keeps clicking EC's button.
  *
  * No effect unless the host sends the actions; registering is safe whenever
  * we're a widget. Returns a teardown function.
@@ -69,10 +86,19 @@ export function startLotusControls(vm: CallViewModel): () => void {
     w.api.transport.reply(ev.detail, {});
     window.dispatchEvent(new Event(LOTUS_TOGGLE_REACTIONS_EVENT));
   };
+  const onSetScreenshare = (ev: CustomEvent<IWidgetApiRequest>): void => {
+    w.api.transport.reply(ev.detail, {});
+    // Call synchronously: the delegated activation is only good for a few
+    // seconds after the message arrived.
+    if (shouldToggleScreenshare(ev.detail.data, vm.sharingScreen$.value)) {
+      vm.toggleScreenSharing?.();
+    }
+  };
 
   w.lazyActions.on(LotusWidgetActions.SetLayout, onSetLayout);
   w.lazyActions.on(LotusWidgetActions.OpenSettings, onOpenSettings);
   w.lazyActions.on(LotusWidgetActions.ToggleReactions, onToggleReactions);
+  w.lazyActions.on(LotusWidgetActions.SetScreenshare, onSetScreenshare);
 
   const sub: Subscription = combineLatest([
     vm.sharingScreen$,
@@ -85,6 +111,7 @@ export function startLotusControls(vm: CallViewModel): () => void {
         ([screensharing, layout]): LotusControlsState => ({
           screensharing,
           layout,
+          screenshareAction: true,
         }),
       ),
       distinctUntilChanged(
@@ -100,5 +127,6 @@ export function startLotusControls(vm: CallViewModel): () => void {
     w.lazyActions.off(LotusWidgetActions.SetLayout, onSetLayout);
     w.lazyActions.off(LotusWidgetActions.OpenSettings, onOpenSettings);
     w.lazyActions.off(LotusWidgetActions.ToggleReactions, onToggleReactions);
+    w.lazyActions.off(LotusWidgetActions.SetScreenshare, onSetScreenshare);
   };
 }