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/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();
+  });
+});