feat(player): draw no volume control where there is no volume

volume-control asks the player whether there is a volume of ours to
control, and renders nothing when there is not.  The decision is in the
control rather than at either mount point because there are two, and
one of them -- the bottom bar's -- lives in index.html, which has no
module scope to make it conditional.

It could not have been a width, and that is the whole design decision.
Every other stand-down rule in this app is keyed on a viewport, because
a width is what a browser can answer and what every tier can test.
This one is a property of the build: keyed on width, an Android tablet
at 600px or more draws the bar's slider over a level the backend has
pinned -- a control that cannot act, on exactly the platform the rule
exists for, which library-status-indicator settled is worse than none.
The same rule is wrong the other way below 600px, where a narrow
desktop window has no hardware keys to fall back on.  index.css keeps
its phone rule, which is now about room and says so.

Rendering nothing and hiding the host are both needed and are separate
assertions: an empty shadow root is what stops a by-role or positional
query finding a button that cannot act, and :host([hidden]) is what
stops the element taking a flex item's worth of the transport.  The
host rule has to be written down, since :host { display: inline-flex }
outranks the UA's [hidden].

Measured at 424x439 by flipping the constant and rebuilding: the album
art goes 39px to 68px and the transport 172px to 143px -- 29px, being
the 21px control plus the 8px gap a hidden box stops drawing.  The
bar's centring is unaffected, since #23's outer columns are the same
min() expression rather than content-sized.

volume-ownership.test.ts is the tier that can exercise the Android
rendering, on an ordinary Linux runner, because the predicate is a
stubbable backend answer.  Both of its tests were confirmed to fail on
the build before this.

Closes #64
Closes #172

e2e/specs/phone-shell.spec.ts

@@ -130,6 +130,25 @@ test.describe('the shell on a phone', () => {
     // are here, and they are the *same* components -- this view
     // composes the transport rather than reimplementing it.
     await expect(app.locator('now-playing-view seek-bar')).toBeVisible();
+
+    // Volume is here **because the player says there is one** (#64),
+    // not because this is a phone. This tier is the platform that owns
+    // its own volume, so what it can assert is that the control's
+    // presence follows that answer -- an inverted polarity in
+    // `volume-style-store` fails here and in `bottom-bar.spec.ts`, and
+    // the *absent* branch is checked in the component tier, where the
+    // binding can be stubbed. Nothing here can reach the Android side.
+    const systemOwns = await app.evaluate(
+      async () =>
+        (await window.__yjEvents.call(
+          'player.Player.SystemOwnsVolume',
+          [],
+          5_000,
+        )) as boolean,
+    );
+
+    expect(systemOwns, 'this platform should own its own volume').toBe(false);
+
     await expect(app.locator('now-playing-view volume-control')).toBeVisible();
 
     // Back goes where the user came from, through the nav stack.

frontend/index.css

@@ -522,16 +522,20 @@ body div.sidebar {
     }
 
     /* Volume stands down here whatever the setting says, because this
-       is about room and about the platform rather than about
-       preference: the hardware keys own volume on a phone, which is
-       also why mediacontrols' Android handler implements no volume
-       callback. It moved from `audio-player`'s own media query when
-       #42 moved the control into the bar — same rule, and now stated
-       where the element actually is.
+       is about room: five controls and a slider do not fit a 360px
+       bar, and the full-screen now-playing view is where seeking and
+       volume go on a phone. It moved from `audio-player`'s own media
+       query when #42 moved the control into the bar — same rule, and
+       now stated where the element actually is.
 
-       `.bottom-bar volume-control`, not the one in
-       `now-playing-view`: that view is the phone's transport and is
-       where a slider does belong. */
+       **This rule used to carry the platform argument too, and no
+       longer does** (#64). "The hardware keys own the volume" is not a
+       width: it is false of a narrow desktop window and true of an
+       Android tablet, which this selector gets backwards both ways.
+       The player answers it now — `SystemOwnsVolume` — and
+       `volume-control` renders nothing when it is true, at every
+       width and in both of its mount points. What is left here is the
+       question a stylesheet can actually answer. */
     .bottom-bar volume-control {
         display: none;
     }

frontend/src/components/audio-player/volume-control/volume-control.ts

@@ -1,4 +1,4 @@
-import { LitElement, html, css } from 'lit';
+import { LitElement, html, css, nothing } from 'lit';
 import { customElement, state } from 'lit/decorators.js';
 import '@awesome.me/webawesome/dist/components/icon/icon.js';
 import '@awesome.me/webawesome/dist/components/slider/slider.js';
@@ -27,6 +27,26 @@ export class VolumeControl extends LitElement {
   @state()
   private popup = volumeStyleStore.popup;
 
+  /**
+   * Whether there is a volume of ours to control at all (#64).
+   *
+   * The decision is made here rather than at either mount point,
+   * because there are two -- the bottom bar's copy lives in
+   * `index.html`, which has no module scope to make it conditional --
+   * and one of them is a control the shell cannot un-render. So the
+   * control answers for itself, and the bar and the phone's
+   * full-screen transport get the same answer without either knowing
+   * the question exists.
+   *
+   * It renders `nothing` *and* hides the host: an empty shadow root is
+   * what stops a positional or role query finding a button that cannot
+   * act, and `:host([hidden])` is what stops the element occupying a
+   * flex item's worth of the transport -- the `:host` display above
+   * outranks the UA's `[hidden]` rule, so it has to be said.
+   */
+  @state()
+  private available = volumeStyleStore.available;
+
   private unsubscribeStyle?: () => void;
 
   // Locally-tracked volume while the user is actively dragging or scrolling.
@@ -43,6 +63,13 @@ export class VolumeControl extends LitElement {
       align-items: center;
     }
 
+    /* See the available field. A gap is only drawn between boxes,
+       so a hidden host costs its parent nothing -- which is where the
+       29px this gives back to Now Playing comes from (#172). */
+    :host([hidden]) {
+      display: none;
+    }
+
     button {
       background: none;
       border: none;
@@ -154,12 +181,15 @@ export class VolumeControl extends LitElement {
 
     this.unsubscribeStyle = volumeStyleStore.subscribe(() => {
       this.popup = volumeStyleStore.popup;
+      this.setAvailable(volumeStyleStore.available);
 
       // Switching to the slider while the popup is open would leave the
       // document listener installed for a popup that no longer renders.
       if (!this.popup) this.closeSlider();
     });
 
+    this.setAvailable(volumeStyleStore.available);
+
     void volumeStyleStore.init();
   }
 
@@ -233,7 +263,25 @@ export class VolumeControl extends LitElement {
   // RENDER
   // ===================================================================
 
+  /**
+   * `hidden` is set imperatively rather than reflected from the state,
+   * because it has to be on the *host* and a `@state` does not reflect.
+   * It is the right attribute besides: it takes the element out of the
+   * accessibility tree as well as out of the layout.
+   */
+  private setAvailable(available: boolean) {
+    this.available = available;
+    this.hidden = !available;
+
+    // A popup left open when the control goes away would keep its
+    // document click listener installed for markup that no longer
+    // renders.
+    if (!available) this.closeSlider();
+  }
+
   override render() {
+    if (!this.available) return nothing;
+
     const muted = this.player.muted;
 
     // Inline, the icon is the mute toggle rather than a disclosure:

frontend/src/components/now-playing-view/now-playing-view.ts

@@ -343,6 +343,12 @@ export class NowPlayingView extends LitElement {
                      a media query because the bottom bar wants a
                      different answer at this same viewport. -->
                 <player-controls context="full"></player-controls>
+                <!-- Rendered unconditionally and absent on its own
+                     terms where the device owns the volume (#64): the
+                     control asks the player, not this view and not the
+                     viewport. A hidden host draws no gap, so that is
+                     29px of a 439px screen back to the album art
+                     (#172). -->
                 <volume-control></volume-control>
             </div>
         `;

frontend/src/store/volume-style-store.ts

@@ -1,5 +1,6 @@
 import { EventsOn } from '@runtime/runtime';
 import { GetPopupVolume } from '@go/config/config.js';
+import { SystemOwnsVolume } from '@go/player/player.js';
 import { Events } from '../events';
 
 type Subscriber = () => void;
@@ -28,10 +29,37 @@ type Subscriber = () => void;
  * becomes one. An install that has chosen the popup sees it swap once
  * on load, which is the cheaper of the two wrong first frames: the
  * inline slider occupies the space the popup's button would have.
+ *
+ * **`available` is the question one step earlier — whether there is a
+ * volume of ours to draw at all (#64).** On Android the hardware keys
+ * are the volume control and the backend pins its own level at
+ * maximum, so a slider here would move nothing.
+ *
+ * It is asked of the *player* rather than of the viewport, and that is
+ * the whole design decision. Every other stand-down rule in this app
+ * is a width, because a width is what a browser can answer and what
+ * every tier can test — but this one is a property of the build. Keyed
+ * on width instead, an Android tablet at 600px or more would draw the
+ * bottom bar's slider over a pinned level: a control that cannot act,
+ * which `library-status-indicator` settled is worse than none.
+ *
+ * It lives beside `popup` because both answer "what presentation does
+ * the volume control get", both readers are the same two components,
+ * and "none" is a presentation. A second store would be a second
+ * subscription in the same `connectedCallback` saying the same thing.
+ *
+ * The initial value is `true` on the same first-frame rule: there is a
+ * volume on every platform but one, and the platform that pins it sees
+ * the control once at boot and never again in the session — the answer
+ * cannot change while the app runs, so by the time the lazily-mounted
+ * now-playing view exists it has long been settled by the bar's own
+ * copy.
  */
 class VolumeStyleStore {
     private value = false;
 
+    private hasVolume = true;
+
     private loaded = false;
 
     private subscribers = new Set<Subscriber>();
@@ -47,13 +75,21 @@ class VolumeStyleStore {
         return this.value;
     }
 
+    /**
+     * Whether this app has a volume of its own to control. False where
+     * the device owns it; see the class comment.
+     */
+    get available(): boolean {
+        return this.hasVolume;
+    }
+
     /** Reads the setting once. Safe to call from every mount. */
     async init(): Promise<void> {
         if (this.loaded) return;
 
         this.loaded = true;
 
-        await this.refresh();
+        await Promise.all([this.refreshAvailability(), this.refresh()]);
     }
 
     subscribe(fn: Subscriber): () => void {
@@ -77,6 +113,28 @@ class VolumeStyleStore {
         }
     }
 
+    /**
+     * Asked once, not on `GeneralConfigChanged`: this is a property of
+     * the platform the binary was built for and cannot change while
+     * the app is running.
+     */
+    private async refreshAvailability(): Promise<void> {
+        try {
+            const owned = await SystemOwnsVolume();
+
+            if (owned === !this.hasVolume) return;
+
+            this.hasVolume = !owned;
+            this.notify();
+        } catch (err) {
+            // The control renders, which is the answer on every
+            // platform but one and is the recoverable way to be wrong:
+            // a working control nobody needs, rather than a missing one
+            // somebody does.
+            console.error('failed to ask who owns the volume', err);
+        }
+    }
+
     private notify(): void {
         for (const fn of this.subscribers) fn();
     }

frontend/test/components/volume-ownership.test.ts

@@ -0,0 +1,93 @@
+/**
+ * Who owns the volume, and what the control does when it is not us
+ * (#64).
+ *
+ * **This is the tier that can exercise the Android branch**, and it is
+ * the reason the predicate is a backend answer rather than a build tag
+ * the frontend cannot see: `SystemOwnsVolume` is a stub here, so the
+ * "no volume" rendering is checked on an ordinary Linux CI runner with
+ * no device anywhere. What no tier here can check is the *constant*
+ * behind it, which `TestPlatformVolumeOwnershipIsDeclaredOncePerPlatform`
+ * sweeps the Go source for instead.
+ *
+ * It is a file of its own because `volumeStyleStore` asks once and
+ * latches — the answer is a property of the binary and cannot change
+ * while the app runs, so there is deliberately no event that refreshes
+ * it. Vitest gives each file its own module registry, which is what
+ * lets the stub be in place before the singleton is first touched.
+ * The *available* case is the rest of `transport.test.ts`, which mounts
+ * the same element under the default stub.
+ */
+import { describe, expect, it, beforeEach } from 'vitest';
+
+import '@components/audio-player/volume-control/volume-control';
+import '@components/now-playing-view/now-playing-view';
+import { Events } from '../../src/events';
+import { emit, resetHarness, stub } from '@test/support/harness';
+import { fixture, shadow, shadowAll } from '@test/support/render';
+import type { TrackInfo } from '@store/player-store';
+
+const TRACK: TrackInfo = {
+  fileName: 'tideline.mp3',
+  filePath: '/music/tideline.mp3',
+  trackLength: 245,
+  seekPosition: 0,
+  state: 'playing',
+  title: 'Tideline',
+  artist: 'Sea Change',
+  album: 'Ebb',
+  coverArt: '',
+  coverArtSmall: '',
+  coverArtMedium: '',
+  coverArtLarge: '',
+  trackChangeId: 1,
+  artistMbid: '',
+  releaseGroupMbid: '',
+  recordingMbid: '',
+};
+
+describe('a platform whose volume we do not own', () => {
+  beforeEach(async () => {
+    resetHarness();
+    stub('player.Player.SystemOwnsVolume', true);
+    stub('config.Config.GetPopupVolume', false);
+
+    // The store latches on the first mount; do it here so every test
+    // below sees a settled answer rather than the first frame.
+    const warm = await fixture('volume-control');
+
+    await warm.updateComplete;
+  });
+
+  it('renders no control at all, and no empty shadow root to find', async () => {
+    const el = await fixture('volume-control');
+
+    await el.updateComplete;
+
+    // Both halves matter. An empty shadow root is what stops a
+    // positional or by-role query finding a button that cannot act;
+    // `hidden` is what stops the host taking a flex item's worth of
+    // space in the transport it sits in.
+    expect(shadowAll(el, 'button')).toHaveLength(0);
+    expect(shadowAll(el, 'wa-slider')).toHaveLength(0);
+    expect(el.hidden, 'the host is not hidden').toBe(true);
+  });
+
+  it('leaves the rest of the phone transport alone', async () => {
+    emit(Events.TrackChanged, TRACK);
+
+    const view = await fixture('now-playing-view');
+
+    await view.updateComplete;
+
+    // Seeking and the transport buttons are not volume, and #64 is
+    // allowed to remove one control, not to thin the screen out.
+    expect(shadow(view, 'seek-bar')).not.toBeNull();
+    expect(shadow(view, 'player-controls')).not.toBeNull();
+
+    const volume = shadow(view, 'volume-control') as HTMLElement | null;
+
+    expect(volume, 'the element is still mounted').not.toBeNull();
+    expect(volume!.hidden, 'a mounted volume-control is not hidden').toBe(true);
+  });
+});