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/index.css

@@ -91,7 +91,7 @@ body.lotus-transparent[data-background="gradient"]::before {
 }
 
 /* [cinny #43] lotusHostControls=1: the host renders its own call bar, so EC's
-footer is hidden but stays in the DOM and in layout-independent position. */
+footer is hidden (kept in the DOM but out of the layout flow). */
 body.lotus-host-controls [data-testid="footer-container"] {
   position: absolute !important;
   visibility: hidden !important;

src/lotus/LotusFrameScreenshare.module.css

@@ -0,0 +1,8 @@
+/* [cinny #43] EC's screenshare button, floating in the frame's bottom-right
+corner while the host draws the rest of the call bar. */
+.floating {
+  position: fixed;
+  right: var(--cpd-space-4x);
+  bottom: calc(env(safe-area-inset-bottom) + var(--cpd-space-4x));
+  z-index: 1;
+}

src/lotus/LotusFrameScreenshare.test.tsx

@@ -0,0 +1,88 @@
+/*
+Copyright 2026 Lotus Guild
+
+SPDX-License-Identifier: AGPL-3.0-only OR LicenseRef-Element-Commercial
+Please see LICENSE in the repository root for full details.
+*/
+
+import { describe, expect, it, vi } from "vitest";
+import { act, render, screen } from "@testing-library/react";
+import userEvent from "@testing-library/user-event";
+import { TooltipProvider } from "@vector-im/compound-web";
+import { BehaviorSubject } from "rxjs";
+
+import { type CallViewModel } from "../state/CallViewModel/CallViewModel";
+import { LotusFrameScreenshare } from "./LotusFrameScreenshare";
+import {
+  lotusFrameScreenshareEnabled,
+  lotusFrameScreenshareVisible$,
+  parseFrameScreensharePayload,
+} from "./lotusControls";
+
+// lotusFlag memoizes the URL params on first read, so set them once here.
+window.history.pushState(
+  {},
+  "",
+  "/?lotusHostControls=1&lotusFrameScreenshare=1",
+);
+
+const mockVm = (
+  toggle: (() => void) | null,
+  sharing = new BehaviorSubject(false),
+): CallViewModel =>
+  ({
+    sharingScreen$: sharing,
+    toggleScreenSharing: toggle,
+  }) as unknown as CallViewModel;
+
+const renderButton = (vm: CallViewModel): void => {
+  render(
+    <TooltipProvider>
+      <LotusFrameScreenshare vm={vm} />
+    </TooltipProvider>,
+  );
+};
+
+describe("LotusFrameScreenshare", () => {
+  it("is enabled by lotusHostControls + lotusFrameScreenshare", () => {
+    expect(lotusFrameScreenshareEnabled()).toBe(true);
+  });
+
+  it("toggles the share from a click inside the frame", async () => {
+    const toggle = vi.fn();
+    renderButton(mockVm(toggle));
+    await userEvent.click(screen.getByTestId("lotus_frame_screenshare"));
+    expect(toggle).toHaveBeenCalledTimes(1);
+  });
+
+  it("follows the sharing state", () => {
+    const sharing = new BehaviorSubject(false);
+    renderButton(mockVm(vi.fn(), sharing));
+    const button = screen.getByTestId("lotus_frame_screenshare");
+    expect(button).toHaveAttribute("aria-checked", "false");
+    act(() => sharing.next(true));
+    expect(button).toHaveAttribute("aria-checked", "true");
+  });
+
+  it("hides while the host's policy forbids sharing, unless a share is live", () => {
+    const sharing = new BehaviorSubject(false);
+    act(() => lotusFrameScreenshareVisible$.next(false));
+    renderButton(mockVm(vi.fn(), sharing));
+    expect(screen.queryByTestId("lotus_frame_screenshare")).toBeNull();
+    act(() => sharing.next(true));
+    expect(screen.getByTestId("lotus_frame_screenshare")).toBeInTheDocument();
+    act(() => lotusFrameScreenshareVisible$.next(true));
+  });
+
+  it("parses the set_frame_screenshare payload", () => {
+    expect(parseFrameScreensharePayload({ visible: false })).toBe(false);
+    expect(parseFrameScreensharePayload({ visible: true })).toBe(true);
+    expect(parseFrameScreensharePayload({})).toBeUndefined();
+    expect(parseFrameScreensharePayload(null)).toBeUndefined();
+  });
+
+  it("renders nothing when screensharing is unavailable", () => {
+    renderButton(mockVm(null));
+    expect(screen.queryByTestId("lotus_frame_screenshare")).toBeNull();
+  });
+});

src/lotus/LotusFrameScreenshare.tsx

@@ -0,0 +1,41 @@
+/*
+Copyright 2026 Lotus Guild
+
+SPDX-License-Identifier: AGPL-3.0-only OR LicenseRef-Element-Commercial
+Please see LICENSE in the repository root for full details.
+*/
+
+import { type FC } from "react";
+
+import { ShareScreenButton } from "../button";
+import { type CallViewModel } from "../state/CallViewModel/CallViewModel";
+import { useBehavior } from "../useBehavior";
+import {
+  lotusFrameScreenshareEnabled,
+  lotusFrameScreenshareVisible$,
+} from "./lotusControls";
+import styles from "./LotusFrameScreenshare.module.css";
+
+/**
+ * [cinny #43] EC's screenshare button, shown inside the frame when the host
+ * can't start a share for us (`lotusFrameScreenshare`). The rest of EC's
+ * footer stays hidden behind the host's call bar.
+ */
+export const LotusFrameScreenshare: FC<{ vm: CallViewModel }> = ({ vm }) => {
+  const sharing = useBehavior(vm.sharingScreen$);
+  const visible = useBehavior(lotusFrameScreenshareVisible$);
+  if (!lotusFrameScreenshareEnabled() || !vm.toggleScreenSharing) return null;
+  // Hidden by the host's call policy, but never while a share is live: the
+  // user must still be able to stop it.
+  if (!visible && !sharing) return null;
+  return (
+    <div className={styles.floating}>
+      <ShareScreenButton
+        size="md"
+        enabled={sharing}
+        onClick={vm.toggleScreenSharing}
+        data-testid="lotus_frame_screenshare"
+      />
+    </div>
+  );
+};

src/lotus/lotusActions.test.ts

@@ -32,6 +32,7 @@ describe("LotusWidgetActions", () => {
       LotusWidgetActions.ToggleReactions,
       LotusWidgetActions.SetScreenshare,
       LotusWidgetActions.SetHotkeys,
+      LotusWidgetActions.SetFrameScreenshare,
     ];
 
     expect(new Set(LOTUS_TO_WIDGET_ACTIONS)).toEqual(new Set(expectedToWidget));

src/lotus/lotusActions.ts

@@ -86,6 +86,12 @@ export enum LotusWidgetActions {
    * to report while focus is inside this frame (cinny #43). `[]` stops.
    */
   SetHotkeys = "io.lotus.set_hotkeys",
+  /**
+   * toWidget: `{ visible: boolean }` — whether the in-frame screenshare button
+   * (`lotusFrameScreenshare`) may show, e.g. false while the room's call
+   * policy forbids screensharing (cinny #43). Defaults to visible.
+   */
+  SetFrameScreenshare = "io.lotus.set_frame_screenshare",
   /**
    * fromWidget: a watched key went down/up here, or this window's focus
    * changed; see `LotusHotkeyReport` (cinny #43).
@@ -93,7 +99,7 @@ export enum LotusWidgetActions {
   Hotkey = "io.lotus.hotkey",
   /**
    * fromWidget: `{ screensharing: boolean, layout: "grid" | "spotlight" | null,
-   * screenshareAction: true, hotkeys: true }`
+   * screenshareAction: true, hotkeys: true, frameScreenshare: boolean }`
    * whenever either changes (cinny #43), so the host stops reading EC's DOM for
    * them. Its arrival also tells the host this fork supports the actions above.
    */
@@ -118,4 +124,5 @@ export const LOTUS_TO_WIDGET_ACTIONS: LotusWidgetActions[] = [
   LotusWidgetActions.ToggleReactions,
   LotusWidgetActions.SetScreenshare,
   LotusWidgetActions.SetHotkeys,
+  LotusWidgetActions.SetFrameScreenshare,
 ];

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,
+    );
   };
 }

src/room/InCallView.tsx

@@ -33,6 +33,7 @@ import { startLotusCallState } from "../lotus/lotusCallState";
 import { startLotusFocus } from "../lotus/lotusFocus";
 import { startLotusControls } from "../lotus/lotusControls";
 import { startLotusHotkeys } from "../lotus/lotusHotkeys";
+import { LotusFrameScreenshare } from "../lotus/LotusFrameScreenshare";
 import { startLotusMicLevel } from "../lotus/lotusMicLevel";
 import { startLotusAudioInject } from "../lotus/lotusAudioInject";
 import { startLotusQuality } from "../lotus/lotusQuality";
@@ -692,6 +693,7 @@ export const InCallView: FC<InCallViewProps> = ({
       {earpieceOverlay}
       <ReactionsOverlay vm={vm} />
       {footer}
+      <LotusFrameScreenshare vm={vm} />
       {showModals && (
         <>
           <RageshakeRequestModal {...rageshakeRequestModalProps} />

src/useTheme.ts

@@ -83,9 +83,9 @@ export const useTheme = (): void => {
       // (The host used to inject `:root { color-scheme }` for this.)
       document.documentElement.style.colorScheme = theme;
     }
-    // [cinny #43] The host draws its own call bar: hide EC's footer, but keep
-    // it in the DOM (the host still clicks its screenshare button on engines
-    // without Capability Delegation). Replaces the host's injected styles.
+    // [cinny #43] The host draws its own call bar: hide EC's footer. Replaces
+    // the host's injected styles. Where the host can't start a share for us,
+    // LotusFrameScreenshare shows EC's screenshare button on its own.
     if (lotusFlag("lotusHostControls"))
       document.body.classList.add("lotus-host-controls");
   }, [previousTheme, requestedTheme]);