Ask one question about whether effects can run

- The menu asked the browser for the APIs; the pipeline asked for those and
  a desktop. A phone browser sat between the two: every effect offered,
  none applied, nothing said.
- Both now ask the same function, so what is offered and what can be
  delivered cannot drift apart again.
- Tested against the lobby, where the switcher stays enabled on a phone —
  in a call it is already withheld there, so a check would pass whatever
  the verdict said. The test fails on the old wiring.
- This also fixes the blur toggle, which had the same split.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

src/components/CallFooterViewModel.test.ts

@@ -14,7 +14,10 @@ import type { CallViewModel } from "../state/CallViewModel/CallViewModel";
 import type { Alignment, Layout } from "../state/layout-types";
 import type { SpotlightTileViewModel } from "../state/TileViewModel";
 import type { DeviceLabel } from "../state/MediaDevices";
-import { createCallFooterViewModel } from "./CallFooterViewModel";
+import {
+  createCallFooterViewModel,
+  createLobbyFooterViewModel,
+} from "./CallFooterViewModel";
 import { HeaderStyle } from "../UrlParams";
 
 const platformMock = vi.hoisted(() => vi.fn(() => "desktop"));
@@ -24,10 +27,12 @@ vi.mock("../Platform", () => ({
   },
 }));
 
-// Prevent supportsBackgroundProcessors from throwing in jsdom – it is not
-// exercised by these tests (only used in `videoToggles`, not `videoOptions`).
+// The SDK's own check needs WebGL and canvas APIs jsdom does not have. The
+// tests below drive it directly, because what they are about is the answer the
+// app gives on top of it.
+const sdkSupportMock = vi.hoisted(() => vi.fn(() => true));
 vi.mock("@livekit/track-processors", () => ({
-  supportsBackgroundProcessors: (): boolean => false,
+  supportsBackgroundProcessors: (): boolean => sdkSupportMock(),
 }));
 
 const outputSelectionMock = vi.hoisted(() => vi.fn(() => true));
@@ -228,4 +233,70 @@ describe("createCallFooterViewModel", () => {
       ]);
     });
   });
+  describe("background effects", () => {
+    // The lobby, deliberately: in a call the whole switcher is already
+    // withheld on a phone, so a check there passes whatever the verdict says.
+    // The lobby keeps its switcher, which is where a phone browser was offered
+    // every effect, given none of them, and told nothing.
+    function lobbyFor(
+      platform: string,
+    ): ReturnType<typeof createLobbyFooterViewModel> {
+      platformMock.mockReturnValue(platform);
+      return createLobbyFooterViewModel(
+        testScope(),
+        mockMuteStates(),
+        twoMicsAndOneCamMediaDevices,
+        /* openSettings */ undefined,
+        /* hangup */ undefined,
+        /* showLogo */ false,
+      );
+    }
+
+    it("offers nothing the pipeline would refuse to honour", () => {
+      sdkSupportMock.mockReturnValue(true);
+      const vm = lobbyFor("ios");
+
+      expect(vm.selectBackgroundEffect$.value).toBeUndefined();
+      expect(vm.toggleBlur$.value).toBeUndefined();
+    });
+
+    it("offers them where the pipeline will honour them", () => {
+      sdkSupportMock.mockReturnValue(true);
+      const vm = lobbyFor("desktop");
+
+      expect(vm.selectBackgroundEffect$.value).toBeDefined();
+      expect(vm.toggleBlur$.value).toBeDefined();
+    });
+
+    it("offers nothing where the browser itself cannot run them", () => {
+      sdkSupportMock.mockReturnValue(false);
+      const vm = lobbyFor("desktop");
+
+      expect(vm.selectBackgroundEffect$.value).toBeUndefined();
+      expect(vm.toggleBlur$.value).toBeUndefined();
+    });
+
+    it("availability is the same before and during a call", () => {
+      for (const supported of [true, false]) {
+        sdkSupportMock.mockReturnValue(supported);
+        platformMock.mockReturnValue("desktop");
+        const inCall = createCallFooterViewModel(
+          testScope(),
+          buildMinimalCallViewModel(gridLayout),
+          mockMuteStates(),
+          twoMicsAndOneCamMediaDevices,
+          /* reactionIdentifier */ undefined,
+          { showControls: true, header: HeaderStyle.Standard },
+        );
+        const lobby = lobbyFor("desktop");
+
+        const offeredInCall =
+          inCall.selectBackgroundEffect$.value !== undefined;
+        const offeredInLobby =
+          lobby.selectBackgroundEffect$.value !== undefined;
+        expect(offeredInLobby).toBe(offeredInCall);
+        expect(offeredInLobby).toBe(supported);
+      }
+    });
+  });
 });

src/components/CallFooterViewModel.tsx

@@ -6,9 +6,10 @@ Please see LICENSE in the repository root for full details.
 */
 
 import { combineLatest, map, type Observable, switchMap } from "rxjs";
-import { supportsBackgroundProcessors } from "@livekit/track-processors";
 import { supportsAudioOutputSelection } from "livekit-client";
 
+import { supportsBackgroundProcessors } from "../livekit/backgroundProcessing";
+
 import { type CallViewModel } from "../state/CallViewModel/CallViewModel";
 import { type MenuOptions } from "./MediaMuteAndSwitchButton";
 import { type MediaDevices } from "../state/MediaDevices";

src/livekit/TrackProcessorContext.tsx

@@ -8,7 +8,6 @@ Please see LICENSE in the repository root for full details.
 import {
   BackgroundProcessorWrapper,
   type ProcessorWrapper,
-  supportsBackgroundProcessors as supportsBackgroundProcessorsLivekitSdk,
   type BackgroundOptions,
   type SwitchBackgroundProcessorOptions,
 } from "@livekit/track-processors";
@@ -40,9 +39,9 @@ import {
   type BackgroundEffect,
 } from "./backgroundEffects";
 import { BackgroundImageStore } from "./backgroundImages";
+import { supportsBackgroundProcessors } from "./backgroundProcessing";
 import { type Behavior } from "../state/Behavior";
 import { type ObservableScope } from "../state/ObservableScope";
-import { platform } from "../Platform";
 
 //TODO-MULTI-SFU: This is not yet fully there.
 // it is a combination of exposing observable and react hooks.
@@ -175,10 +174,6 @@ interface Props {
   children: JSX.Element;
 }
 
-function supportsBackgroundProcessors(): boolean {
-  return supportsBackgroundProcessorsLivekitSdk() && platform === "desktop";
-}
-
 /** Translates a chosen effect into the pipeline's own vocabulary. */
 function switchOptionsFor(
   effect: BackgroundEffect,

src/livekit/backgroundProcessing.ts

@@ -0,0 +1,27 @@
+/*
+Copyright 2026 Element Creations Ltd.
+
+SPDX-License-Identifier: AGPL-3.0-only OR LicenseRef-Element-Commercial
+Please see LICENSE in the repository root for full details.
+*/
+
+import { supportsBackgroundProcessors as supportsBackgroundProcessorsLivekitSdk } from "@livekit/track-processors";
+
+import { platform } from "../Platform";
+
+/**
+ * Whether this device can run background effects at all.
+ *
+ * The one answer, for everything that asks. The controls and the pipeline used
+ * to work it out separately, and differently: the pipeline required a desktop
+ * while the controls asked only whether the browser had the APIs. A phone
+ * browser has them, so it was offered a choice the pipeline then refused to
+ * honour, and the video simply never changed — the worst of both, since it
+ * neither worked nor said why.
+ *
+ * Anything that offers a background effect must ask this, so that what is
+ * offered and what can be delivered cannot drift apart again.
+ */
+export function supportsBackgroundProcessors(): boolean {
+  return supportsBackgroundProcessorsLivekitSdk() && platform === "desktop";
+}