feat(lotus): in-frame screenshare button where the host can't delegate (cinny #43)

Firefox, Safari and WebKitGTK have no Capability Delegation, so the host
can't start a share from its call bar: getDisplayMedia needs the click in
this frame. Until now the host clicked EC's hidden footer button through
the DOM, which only works while the frame is same-origin.

With lotusHostControls + lotusFrameScreenshare the fork now shows EC's own
screenshare button (floating bottom-right, footer still hidden), so the
click is native. controls_state reports frameScreenshare so the host hides
its own button. New toWidget io.lotus.set_frame_screenshare { visible }
lets the host hide it while the room's call policy forbids screensharing
(never while a share is live, so it can still be stopped).

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

src/lotus/lotusControls.ts

@@ -6,13 +6,13 @@ Please see LICENSE in the repository root for full details.
 */
 
 import { type IWidgetApiRequest } from "matrix-widget-api";
-import { combineLatest, of, type Subscription } from "rxjs";
+import { BehaviorSubject, combineLatest, of, type Subscription } from "rxjs";
 import { distinctUntilChanged, map, switchMap } from "rxjs/operators";
 
 import { type CallViewModel } from "../state/CallViewModel/CallViewModel";
 import { type LayoutMode } from "../state/LayoutSwitchViewModel";
 import { widget } from "../widget";
-import { LotusWidgetActions, lotusSendToHost } from "./lotusWidget";
+import { LotusWidgetActions, lotusFlag, lotusSendToHost } from "./lotusWidget";
 
 /** Window event the reactions button listens for (see ReactionToggleButton). */
 export const LOTUS_TOGGLE_REACTIONS_EVENT = "lotus:toggle-reactions";
@@ -25,6 +25,38 @@ export interface LotusControlsState {
   screenshareAction: true;
   /** This fork handles `io.lotus.set_hotkeys` (always true). */
   hotkeys: true;
+  /**
+   * EC's own screenshare button is shown inside the frame (see
+   * `lotusFrameScreenshare`), so the host should hide its own.
+   */
+  frameScreenshare: boolean;
+}
+
+/**
+ * [cinny #43] `lotusFrameScreenshare=1` (with `lotusHostControls`): the host
+ * can't hand the user's click into this frame (no Capability Delegation:
+ * Firefox, Safari, WebKitGTK), and `getDisplayMedia` needs that click here.
+ * So EC's own screenshare button stays visible in the frame, where the click
+ * is native, instead of the host clicking it through the DOM.
+ */
+export function lotusFrameScreenshareEnabled(): boolean {
+  return lotusFlag("lotusHostControls") && lotusFlag("lotusFrameScreenshare");
+}
+
+/**
+ * Host-controlled visibility of the in-frame screenshare button
+ * (`io.lotus.set_frame_screenshare`): the host hides it while the room's
+ * call policy forbids screensharing, as it does its own call-bar button.
+ */
+export const lotusFrameScreenshareVisible$ = new BehaviorSubject(true);
+
+/** `{ visible }` payload → the visibility to apply, or undefined. Exported for tests. */
+export function parseFrameScreensharePayload(
+  data: unknown,
+): boolean | undefined {
+  if (typeof data !== "object" || data === null) return undefined;
+  const { visible } = data as { visible?: unknown };
+  return typeof visible === "boolean" ? visible : undefined;
 }
 
 /** `{ layout }` payload → a layout mode, or undefined if invalid. Exported for tests. */
@@ -100,7 +132,17 @@ export function startLotusControls(vm: CallViewModel): () => void {
   w.lazyActions.on(LotusWidgetActions.SetLayout, onSetLayout);
   w.lazyActions.on(LotusWidgetActions.OpenSettings, onOpenSettings);
   w.lazyActions.on(LotusWidgetActions.ToggleReactions, onToggleReactions);
+  const onSetFrameScreenshare = (ev: CustomEvent<IWidgetApiRequest>): void => {
+    w.api.transport.reply(ev.detail, {});
+    const visible = parseFrameScreensharePayload(ev.detail.data);
+    if (visible !== undefined) lotusFrameScreenshareVisible$.next(visible);
+  };
+
   w.lazyActions.on(LotusWidgetActions.SetScreenshare, onSetScreenshare);
+  w.lazyActions.on(
+    LotusWidgetActions.SetFrameScreenshare,
+    onSetFrameScreenshare,
+  );
 
   const sub: Subscription = combineLatest([
     vm.sharingScreen$,
@@ -115,6 +157,7 @@ export function startLotusControls(vm: CallViewModel): () => void {
           layout,
           screenshareAction: true,
           hotkeys: true,
+          frameScreenshare: lotusFrameScreenshareEnabled(),
         }),
       ),
       distinctUntilChanged(
@@ -131,5 +174,9 @@ export function startLotusControls(vm: CallViewModel): () => void {
     w.lazyActions.off(LotusWidgetActions.OpenSettings, onOpenSettings);
     w.lazyActions.off(LotusWidgetActions.ToggleReactions, onToggleReactions);
     w.lazyActions.off(LotusWidgetActions.SetScreenshare, onSetScreenshare);
+    w.lazyActions.off(
+      LotusWidgetActions.SetFrameScreenshare,
+      onSetFrameScreenshare,
+    );
   };
 }