Draw the microphone level without re-rendering

- The level state carries a Behavior, so the state changes only when its kind
  does.
- The meter renders its bars once and an effect draws the level into them.
- The render-counting test now counts every commit, and asserts none while the
  level moves.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

src/components/MediaMuteAndSwitchButton.test.tsx

@@ -15,24 +15,16 @@ import {
   type RenderResult,
 } from "@testing-library/react";
 import userEvent from "@testing-library/user-event";
-import { type JSX, useState, type ReactNode } from "react";
+import { Profiler, type JSX, useState, type ReactNode } from "react";
 import { TooltipProvider } from "@vector-im/compound-web";
 
 import {
   MediaMuteAndSwitchButton,
   type MenuOptions,
 } from "./MediaMuteAndSwitchButton";
-import { MediaDevicesContext, useMediaDevices } from "../MediaDevicesContext";
+import { MediaDevicesContext } from "../MediaDevicesContext";
 import { type MediaDevices } from "../state/MediaDevices";
 import { restoreAudioCapture, stubAudioCapture } from "../utils/test";
-import type * as MediaDevicesContextModule from "../MediaDevicesContext";
-
-// The menu reads the devices on every render and the meter never does, so
-// these calls count the menu's own renders.
-vi.mock("../MediaDevicesContext", async (importOriginal) => {
-  const actual = await importOriginal<typeof MediaDevicesContextModule>();
-  return { ...actual, useMediaDevices: vi.fn(actual.useMediaDevices) };
-});
 
 interface RenderOptions {
   requestDeviceNames: () => void;
@@ -488,15 +480,19 @@ describe("MediaMuteAndSwitchButton", () => {
     ).not.toHaveAttribute("aria-disabled", "true");
   });
 
-  test("redraws the meter and not the device rows around it", async () => {
-    // The level is held by the meter, so a moving level doesn't re-render the
-    // menu.
+  test("moves the level without re-rendering anything", async () => {
+    // The level is drawn into the DOM, so a moving level commits nothing.
     const capture = stubAudioCapture();
     const user = userEvent.setup();
-    const menuRenders = vi.mocked(useMediaDevices);
+    let commits = 0;
 
     const { getByRole } = renderComponent(
-      <>
+      <Profiler
+        id="menu"
+        onRender={(): void => {
+          commits++;
+        }}
+      >
         <MediaMuteAndSwitchButton
           iconsAndLabels="audio"
           enabled={true}
@@ -513,13 +509,16 @@ describe("MediaMuteAndSwitchButton", () => {
           selectedOutputOption="spk1"
           onSelectOutput={vi.fn()}
         />
-      </>,
+      </Profiler>,
     );
 
     await user.click(getByRole("button", { name: "Microphone" }));
     capture.grant();
+    await vi.waitFor(() => expect(capture.contexts).toHaveLength(1));
     const meter = await screen.findByRole("meter");
-    const settled = menuRenders.mock.calls.length;
+    // The capture's arrival is one render: the idle level swapped for its own.
+    await act(async () => {});
+    const settled = commits;
 
     // The meter smooths by elapsed time, so hand-driven frames need a clock.
     let elapsed = performance.now();
@@ -538,7 +537,7 @@ describe("MediaMuteAndSwitchButton", () => {
     clock.mockRestore();
 
     expect(meter.getAttribute("aria-valuenow")).not.toBe("0");
-    expect(menuRenders.mock.calls.length - settled).toBe(0);
+    expect(commits - settled).toBe(0);
   });
 
   test("camera menu uses the same selection pattern and keeps the blur toggle", async () => {

src/components/MicrophoneLevelMeter.stories.tsx

@@ -15,6 +15,7 @@ import {
 } from "./MicrophoneLevelMeter";
 import styles from "./MicrophoneLevelMeter.module.css";
 import { LEVEL_SCALE } from "../state/MicrophoneLevel";
+import { constant } from "../state/Behavior";
 
 /** Roughly the menu's width. It only decides how many bars fit. */
 const STORY_WIDTH = 256;
@@ -41,18 +42,18 @@ type Story = StoryObj<typeof meta>;
 
 /** A quiet room: hiss below the noise floor lights nothing. */
 export const Silent: Story = {
-  args: { state: { type: "level", level: 0 } },
+  args: { state: { type: "level", level: constant(0) } },
   play: async ({ canvasElement }) => {
     await expect(litSegments(canvasElement)).toBe(0);
   },
 };
 
 export const QuietSpeech: Story = {
-  args: { state: { type: "level", level: 5 } },
+  args: { state: { type: "level", level: constant(5) } },
 };
 
 export const NormalSpeech: Story = {
-  args: { state: { type: "level", level: 12 } },
+  args: { state: { type: "level", level: constant(12) } },
   play: async ({ canvasElement }) => {
     // A floor, not a count: the count follows from the design's bar and gap sizes.
     await expect(
@@ -62,16 +63,20 @@ export const NormalSpeech: Story = {
 };
 
 export const LoudSpeech: Story = {
-  args: { state: { type: "level", level: LEVEL_SCALE } },
+  args: { state: { type: "level", level: constant(LEVEL_SCALE) } },
 };
 
 /** The three volumes differ in how many bars are lit, not only in colour. */
 export const VolumesAreDistinguishable: Story = {
-  args: { state: { type: "level", level: 5 } },
+  args: { state: { type: "level", level: constant(5) } },
   play: async ({ canvasElement, mount }) => {
     const lit: number[] = [];
     for (const level of [5, 12, LEVEL_SCALE]) {
-      await mount(<MicrophoneLevelMeter state={{ type: "level", level }} />);
+      await mount(
+        <MicrophoneLevelMeter
+          state={{ type: "level", level: constant(level) }}
+        />,
+      );
       lit.push(litSegments(canvasElement));
       await expect(within(canvasElement).getByRole("meter")).toHaveAttribute(
         "aria-valuenow",
@@ -116,7 +121,7 @@ export const NoDevice: Story = {
 
 /** The same meter at two widths: the bars keep their size and only their count changes. */
 export const ShapeStaysTheSameAtAnyWidth: Story = {
-  args: { state: { type: "level", level: 12 } },
+  args: { state: { type: "level", level: constant(12) } },
   play: async ({ mount, args }) => {
     const narrow = await measureAt(mount, args, 180);
     const wide = await measureAt(mount, args, 400);

src/components/MicrophoneLevelMeter.test.tsx

@@ -10,10 +10,13 @@ import { render, screen } from "@testing-library/react";
 
 import { MicrophoneLevelMeter } from "./MicrophoneLevelMeter";
 import { LEVEL_SCALE } from "../state/MicrophoneLevel";
+import { constant } from "../state/Behavior";
 
 describe("MicrophoneLevelMeter", () => {
   test("announces the level rather than relying on hue", () => {
-    render(<MicrophoneLevelMeter state={{ type: "level", level: 6 }} />);
+    render(
+      <MicrophoneLevelMeter state={{ type: "level", level: constant(6) }} />,
+    );
 
     const meter = screen.getByRole("meter", { name: "Microphone level" });
     expect(meter).toHaveAttribute("aria-valuenow", "6");

src/components/MicrophoneLevelMeter.tsx

@@ -5,7 +5,14 @@ SPDX-License-Identifier: AGPL-3.0-only OR LicenseRef-Element-Commercial
 Please see LICENSE in the repository root for full details.
 */
 
-import { useCallback, useState, type FC, type Ref } from "react";
+import {
+  useCallback,
+  useLayoutEffect,
+  useRef,
+  useState,
+  type FC,
+  type Ref,
+} from "react";
 import { Text } from "@vector-im/compound-web";
 import { MicOnIcon } from "@vector-im/compound-design-tokens/assets/web/icons";
 import classNames from "classnames";
@@ -57,20 +64,45 @@ export const MicrophoneLevelMeter: FC<MicrophoneLevelMeterProps> = ({
   // Bars keep one size, so their count follows the width. Starts full so the
   // first paint, and jsdom, draw a whole meter.
   const [barCount, setBarCount] = useState(LEVEL_SCALE);
+  const segments = useRef<HTMLDivElement | null>(null);
   const track = useCallback(
     (element: HTMLDivElement | null): (() => void) | undefined => {
       if (element === null) return;
+      segments.current = element;
       const subscription = observeElementSize$(element)
         .pipe(
           map(({ width }) => barsThatFit(element, width)),
           distinctUntilChanged(),
         )
         .subscribe(setBarCount);
-      return (): void => subscription.unsubscribe();
+      return (): void => {
+        subscription.unsubscribe();
+        segments.current = null;
+      };
     },
     [],
   );
 
+  // Drawn straight into the DOM: the level changes many times a second, and
+  // re-rendering for each change is what this avoids.
+  useLayoutEffect(() => {
+    const element = segments.current;
+    if (state.type !== "level" || element === null) return;
+    const subscription = state.level.subscribe((level) => {
+      element.setAttribute("aria-valuenow", String(level));
+      element.setAttribute(
+        "aria-valuetext",
+        t("microphone_level.value", { level, max: LEVEL_SCALE }),
+      );
+      // The level is a share of the scale, not a bar count.
+      const lit = Math.round((level / LEVEL_SCALE) * barCount);
+      Array.from(element.children).forEach((bar, i) =>
+        bar.classList.toggle(styles.segmentLit, i < lit),
+      );
+    });
+    return (): void => subscription.unsubscribe();
+  }, [state, barCount, t]);
+
   return (
     <div ref={ref} className={classNames(styles.meter, className)}>
       <MicOnIcon className={styles.icon} aria-hidden />
@@ -88,22 +120,15 @@ export const MicrophoneLevelMeter: FC<MicrophoneLevelMeterProps> = ({
           aria-label={t("microphone_level.label")}
           aria-valuemin={0}
           aria-valuemax={LEVEL_SCALE}
-          aria-valuenow={state.level}
+          // The first paint's value; the effect above keeps it current.
+          aria-valuenow={state.level.value}
           aria-valuetext={t("microphone_level.value", {
-            level: state.level,
+            level: state.level.value,
             max: LEVEL_SCALE,
           })}
         >
           {Array.from({ length: barCount }, (_, i) => (
-            <span
-              key={i}
-              aria-hidden
-              className={classNames(styles.segment, {
-                // The level is a share of the scale, not a bar count.
-                [styles.segmentLit]:
-                  i < Math.round((state.level / LEVEL_SCALE) * barCount),
-              })}
-            />
+            <span key={i} aria-hidden className={styles.segment} />
           ))}
         </div>
       )}

src/components/useMicrophoneLevel.test.tsx

@@ -35,6 +35,7 @@ describe("useMicrophoneLevel", () => {
     rerender({ id: "mic2" });
 
     // No level carried over from the previous device.
-    expect(result.current).toEqual({ type: "level", level: 0 });
+    const state = result.current;
+    expect(state.type === "level" ? state.level.value : state.type).toBe(0);
   });
 });

src/components/useMicrophoneLevel.ts

@@ -11,8 +11,9 @@ import {
   type MicrophoneState,
   observeMicrophoneState$,
 } from "../state/MicrophoneLevel";
+import { constant } from "../state/Behavior";
 
-const IDLE: MicrophoneState = { type: "level", level: 0 };
+const IDLE: MicrophoneState = { type: "level", level: constant(0) };
 
 /** The live level of a microphone, captured only while `active`. */
 export function useMicrophoneLevel(

src/state/MicrophoneLevel.test.ts

@@ -157,16 +157,20 @@ describe("observeMicrophoneState$", () => {
   test("says nothing on a frame that did not change the level", async () => {
     const capture = stubAudioCapture();
 
-    let emissions = 0;
-    const subscription = observeMicrophoneState$("mic1").subscribe(
-      () => emissions++,
-    );
+    const states: string[] = [];
+    let levels = 0;
+    const subscription = observeMicrophoneState$("mic1").subscribe((state) => {
+      states.push(state.type);
+      if (state.type === "level") state.level.subscribe(() => levels++);
+    });
     capture.grant();
-    await vi.waitFor(() => expect(emissions).toBe(1));
+    await vi.waitFor(() => expect(levels).toBe(1));
 
-    // Read every frame, but a steady signal emits once.
+    // Read every frame, but a steady signal emits once, and the state itself
+    // arrives only once.
     capture.drawFrames(20);
-    expect(emissions).toBe(1);
+    expect(levels).toBe(1);
+    expect(states).toEqual(["level"]);
 
     subscription.unsubscribe();
   });

src/state/MicrophoneLevel.ts

@@ -5,12 +5,15 @@ SPDX-License-Identifier: AGPL-3.0-only OR LicenseRef-Element-Commercial
 Please see LICENSE in the repository root for full details.
 */
 
-import { distinctUntilChanged, Observable } from "rxjs";
+import { BehaviorSubject, Observable } from "rxjs";
 import { logger } from "matrix-js-sdk/lib/logger";
 
+import { type Behavior } from "./Behavior";
+
 /** What the microphone picks up, or why it can't be read. */
 export type MicrophoneState =
-  | { type: "level"; level: number }
+  // A Behavior, so a changing level can be drawn without re-rendering.
+  | { type: "level"; level: Behavior<number> }
   | { type: "permission-denied" }
   | { type: "no-device" };
 
@@ -29,6 +32,7 @@ export function observeMicrophoneState$(
     let stream: MediaStream | undefined;
     let context: AudioContext | undefined;
     let frame: number | undefined;
+    let level: BehaviorSubject<number> | undefined;
 
     // Idempotent: teardown and start can both call it.
     const release = (): void => {
@@ -38,6 +42,8 @@ export function observeMicrophoneState$(
       frame = undefined;
       stream = undefined;
       context = undefined;
+      level?.complete();
+      level = undefined;
     };
 
     const start = async (): Promise<void> => {
@@ -59,6 +65,9 @@ export function observeMicrophoneState$(
       const samples = new Uint8Array(analyser.fftSize);
       let displayed = 0;
       let previousFrame = performance.now();
+      const current = new BehaviorSubject(0);
+      level = current;
+      subscriber.next({ type: "level", level: current });
 
       const read = (): void => {
         analyser.getByteTimeDomainData(samples);
@@ -75,7 +84,9 @@ export function observeMicrophoneState$(
           now - previousFrame,
         );
         previousFrame = now;
-        subscriber.next({ type: "level", level: segmentsForVolume(displayed) });
+        // Frames that don't move the quantised level say nothing.
+        const next = segmentsForVolume(displayed);
+        if (next !== current.value) current.next(next);
         frame = requestAnimationFrame(read);
       };
       read();
@@ -88,14 +99,7 @@ export function observeMicrophoneState$(
     });
 
     return release;
-  }).pipe(
-    // Frames that don't move the quantised level don't reach React.
-    distinctUntilChanged(
-      (a, b) =>
-        a.type === b.type &&
-        (a.type !== "level" || b.type !== "level" || a.level === b.level),
-    ),
-  );
+  });
 }
 
 /** What a failure to open the microphone means for the person using it. */