fix(a11y): make Settings and the Downloads tabs keyboard-reachable

a11y.1 is the audit's last Critical and reproduced exactly: seven
config-section headers, seven bare `<div @click>`s with no tabindex,
no role and no aria-expanded, and every section collapsed by default —
so every setting in the app was behind a control that could not be
tabbed to. a11y.2 is the same bug in Downloads' two `<div class=tab>`s.

Both now follow patterns the app already had: a real
`<button aria-expanded aria-controls>` (explore-artist-details has five),
and a role=tablist/tab/tabpanel with a roving tab stop and
Left/Right/Home/End. The section body renders unconditionally and is
toggled with `hidden`, because aria-controls has to name an element
that exists and the slot's light-DOM children exist either way.

H-22's reorder ships with them: Libraries is first and the only
expanded section, Search Index — configured once, if ever — is second
to last. The Playback/Audio section H-22 also asks for is deliberately
not here: there is no output-device, gapless, crossfade or replay-gain
setting in backend/config to expose, and a section of controls that do
nothing is worse than admitting it does not exist.

Settings also stops advertising `tracklist.delete`, which was bound to
Delete and configurable in the UI while nothing listened for the event
it dispatched.

backend/shortcuts/config.go

@@ -32,9 +32,15 @@ func DefaultBindings() map[string]string {
 		// App actions (Global scope, Ctrl modifier)
 		"app.selectAll": "Ctrl+A",
 
-		// Panel-specific (track list)
+		// Panel-specific (track list).  There is no `tracklist.delete`:
-		"tracklist.play":   "Enter",
+		// it was bound to Delete and advertised in Settings as
-		"tracklist.delete": "Delete",
+		// configurable while nothing listened for it, because "remove
+		// from library" does not exist and it is not clear what it would
+		// remove — the row (which the next scan puts back unless the path
+		// is also excluded) or the file (a delete-your-music button one
+		// keystroke from a focused row).  Advertise it again when it does
+		// something.
+		"tracklist.play": "Enter",
 
 		// Panel-specific (autotag review).  These are the keys the
 		// autotag page used to bind on its own document listener, which

frontend/src/components/config-page/config-page.ts

@@ -163,12 +163,6 @@ export class ConfigPage extends ViewLifecycleMixin(LitElement) {
             scope: 'panel:track-list',
             defaultKey: 'Enter',
         },
-        'tracklist.delete': {
-            label: 'Remove Selected',
-            category: 'Navigation',
-            scope: 'panel:track-list',
-            defaultKey: 'Delete',
-        },
         'autotag.apply': {
             label: 'Apply Match',
             category: 'Autotag',
@@ -1500,14 +1494,20 @@ export class ConfigPage extends ViewLifecycleMixin(LitElement) {
         return html`
             <h2>Settings</h2>
 
-            ${this.renderSearchSection()}
+            <!--
+              Ordered by how often a setting is *reached*, not by how
+              the sections were written (H-22).  Libraries was last and
+              below the fold while Search Index — which is configured
+              once, if ever — was first and the only expanded one.
+            -->
+            ${this.renderLibrarySection()}
             ${this.renderNowPlayingSection()}
             ${this.renderThemeSection()}
-            ${this.renderFavoritesSection()}
             ${this.renderTrackListSection()}
+            ${this.renderFavoritesSection()}
             ${this.renderShortcutsSection()}
+            ${this.renderSearchSection()}
             <download-clients></download-clients>
-            ${this.renderLibrarySection()}
         `;
     }
 
@@ -1520,7 +1520,6 @@ export class ConfigPage extends ViewLifecycleMixin(LitElement) {
             <config-section
                 heading="Search Index"
                 description="The explore search index is built from the MusicBrainz/ListenBrainz data dumps — popular artists, albums, and tracks with listen counts — for fast offline search."
-                .open=${true}
             >
                 <div class="index-status">
                     ${s
@@ -2035,6 +2034,7 @@ export class ConfigPage extends ViewLifecycleMixin(LitElement) {
                 heading="Libraries"
                 description="Manage your music library folders. Scanning and its
                     progress live in the Jobs panel."
+                .open=${true}
             >
                 <div class="scan-actions">
                     <button

frontend/src/components/config-page/config-section.ts

@@ -5,6 +5,18 @@ import { customElement, property, state } from 'lit/decorators.js';
  * A visual grouping wrapper for config fields.
  * Renders a collapsible heading with optional description,
  * and a slot for fields. Starts collapsed by default.
+ *
+ * The header is a real `<button aria-expanded aria-controls>`, which
+ * it was not: it was a bare `<div @click>` with no tabindex and no
+ * role, and every section defaults to collapsed — so every setting in
+ * the app sat behind a control that could not be tabbed to (a11y.1).
+ * `explore-artist-details` has had the correct pattern in five places
+ * the whole time.
+ *
+ * The body renders unconditionally and is toggled with `hidden`,
+ * rather than being added and removed. `aria-controls` has to name an
+ * element that exists, and the slot's light-DOM children exist either
+ * way — a conditional `<slot>` only stops projecting them.
  */
 @customElement('config-section')
 export class ConfigSection extends LitElement {
@@ -24,9 +36,15 @@ export class ConfigSection extends LitElement {
             display: flex;
             align-items: flex-start;
             gap: 0.5em;
+            width: 100%;
             padding: 1.25em;
             cursor: pointer;
             user-select: none;
+            background: none;
+            border: none;
+            font: inherit;
+            color: inherit;
+            text-align: left;
         }
 
         .header:hover {
@@ -34,6 +52,12 @@ export class ConfigSection extends LitElement {
             border-radius: 6px;
         }
 
+        .header:focus-visible {
+            outline: 2px solid var(--yj-accent, #ffc107);
+            outline-offset: -2px;
+            border-radius: 6px;
+        }
+
         .chevron {
             flex-shrink: 0;
             width: 16px;
@@ -69,6 +93,10 @@ export class ConfigSection extends LitElement {
             padding: 0 1.25em 1.25em;
         }
 
+        .body[hidden] {
+            display: none;
+        }
+
         .fields {
             display: flex;
             flex-direction: column;
@@ -99,8 +127,11 @@ export class ConfigSection extends LitElement {
     override render() {
         return html`
             <div class="section">
-                <div
+                <button
+                    type="button"
                     class="header"
+                    aria-expanded=${this.expanded ? 'true' : 'false'}
+                    aria-controls="section-body"
                     @click=${this.toggle}
                 >
                     <svg
@@ -120,16 +151,12 @@ export class ConfigSection extends LitElement {
                               </p>`
                             : nothing}
                     </div>
+                </button>
+                <div class="body" id="section-body" ?hidden=${!this.expanded}>
+                    <div class="fields">
+                        <slot></slot>
+                    </div>
                 </div>
-                ${this.expanded
-                    ? html`
-                          <div class="body">
-                              <div class="fields">
-                                  <slot></slot>
-                              </div>
-                          </div>
-                      `
-                    : nothing}
             </div>
         `;
     }

frontend/src/components/downloads-view/downloads-view.ts

@@ -98,10 +98,18 @@ export class DownloadsView extends ViewLifecycleMixin(LitElement) {
                 font-weight: 600;
                 color: var(--yj-text-secondary, #b3b3b3);
                 cursor: pointer;
+                border: none;
                 border-bottom: 2px solid transparent;
+                background: none;
+                font-family: inherit;
                 user-select: none;
             }
 
+            .tab:focus-visible {
+                outline: 2px solid var(--yj-accent, #ffd43b);
+                outline-offset: -2px;
+            }
+
             .tab:hover {
                 color: var(--yj-text-primary, #fff);
             }
@@ -261,25 +269,79 @@ export class DownloadsView extends ViewLifecycleMixin(LitElement) {
                 everything on the list immediately.
             </p>
 
-            <div class="tabs">
+            <div
-                <div
+                class="tabs"
-                    class="tab ${this.tab === 'requests' ? 'active' : ''}"
+                role="tablist"
-                    @click=${() => (this.tab = 'requests')}
+                aria-label="Downloads sections"
-                >
+                @keydown=${this.onTabKeydown}
-                    Requests
+            >
-                </div>
+                ${DownloadsView.TABS.map(
-                <div
+                    ([id, label]) => html`
-                    class="tab ${this.tab === 'downloads' ? 'active' : ''}"
+                        <button
-                    @click=${() => (this.tab = 'downloads')}
+                            type="button"
-                >
+                            role="tab"
-                    Downloads
+                            id=${`tab-${id}`}
-                </div>
+                            class="tab ${this.tab === id ? 'active' : ''}"
+                            aria-selected=${this.tab === id ? 'true' : 'false'}
+                            aria-controls=${`panel-${id}`}
+                            tabindex=${this.tab === id ? 0 : -1}
+                            @click=${() => (this.tab = id)}
+                        >
+                            ${label}
+                        </button>
+                    `,
+                )}
             </div>
 
-            ${this.tab === 'requests' ? this.renderRequests() : this.renderDownloads()}
+            <div
+                role="tabpanel"
+                id=${`panel-${this.tab}`}
+                aria-labelledby=${`tab-${this.tab}`}
+            >
+                ${this.tab === 'requests' ? this.renderRequests() : this.renderDownloads()}
+            </div>
         `;
     }
 
+    /**
+     * The tabs, in order, so the markup and the keyboard model read
+     * the same list rather than each spelling it out (a11y.2: these
+     * were two `<div @click>`s with no roles, no tabindex and no
+     * keyboard path at all, which made the Downloads half of the
+     * Downloads view mouse-only).
+     */
+    private static readonly TABS: ReadonlyArray<readonly [Tab, string]> = [
+        ['requests', 'Requests'],
+        ['downloads', 'Downloads'],
+    ];
+
+    /**
+     * A tablist moves with Left/Right/Home/End and activates as it
+     * moves — the panel is already rendered, so there is nothing to
+     * defer. Focus follows, which is what makes the roving tabindex
+     * mean anything.
+     */
+    private onTabKeydown = (e: KeyboardEvent): void => {
+        const ids = DownloadsView.TABS.map(([id]) => id);
+        const at = ids.indexOf(this.tab);
+
+        let next: number | null = null;
+        if (e.key === 'ArrowRight') next = (at + 1) % ids.length;
+        else if (e.key === 'ArrowLeft') next = (at - 1 + ids.length) % ids.length;
+        else if (e.key === 'Home') next = 0;
+        else if (e.key === 'End') next = ids.length - 1;
+
+        if (next === null) return;
+
+        e.preventDefault();
+        this.tab = ids[next]!;
+        void this.updateComplete.then(() => {
+            this.shadowRoot
+                ?.querySelector<HTMLButtonElement>(`#tab-${this.tab}`)
+                ?.focus();
+        });
+    };
+
     // -----------------------------------------------------------------
     // Requests tab
     // -----------------------------------------------------------------

frontend/src/services/keyboard-shortcut-service.ts

@@ -411,11 +411,9 @@ async function dispatch(action: string): Promise<void> {
             );
             break;
 
-        case 'tracklist.delete':
+        // No `tracklist.delete`: it dispatched an event nothing
-            document.dispatchEvent(
+        // listened for, from a binding Settings advertised as
-                new CustomEvent('shortcut:tracklist-delete'),
+        // configurable.  See backend/shortcuts/config.go.
-            );
-            break;
 
         // Panel-specific: autotag review.  The view listens for these
         // while it is the view on screen, and for nothing while it is

frontend/test/components/settings-reach.test.ts

@@ -0,0 +1,105 @@
+/**
+ * Settings is reachable, and Downloads' tabs are tabs.
+ *
+ * `a11y.1` is the last Critical in the accessibility audit: every
+ * `config-section` header was a bare `<div @click>` with no tabindex,
+ * no role and no `aria-expanded`, and every section defaults to
+ * collapsed — so every setting in the app sat behind a control that
+ * could not be tabbed to. `a11y.2` is the same bug one page over, in
+ * Downloads' two `<div class="tab">`s.
+ *
+ * Reproduced in the running app before either was fixed: seven
+ * sections, seven `DIV`s, `tabindex` and `role` null on all of them.
+ */
+import { describe, expect, it } from 'vitest';
+
+import '@components/config-page/config-section';
+import '@components/downloads-view/downloads-view';
+import { fixture, shadow, shadowAll, update } from '@test/support/render';
+
+describe('<config-section> disclosure', () => {
+  it('is a button that reports its state', async () => {
+    const el = await fixture('config-section', { heading: 'Libraries' });
+
+    const header = shadow(el, '.header');
+
+    expect(header?.tagName).toBe('BUTTON');
+    expect(header?.getAttribute('aria-expanded')).toBe('false');
+  });
+
+  it('points aria-controls at a body that exists while collapsed', async () => {
+    const el = await fixture('config-section', { heading: 'Theme' });
+
+    const id = shadow(el, '.header')?.getAttribute('aria-controls');
+    const body = shadow(el, `#${id}`);
+
+    // The body renders unconditionally and is toggled with `hidden`:
+    // aria-controls has to name an element that is in the DOM, and the
+    // slot's light-DOM children exist either way.
+    expect(body).toBeTruthy();
+    expect((body as HTMLElement).hidden).toBe(true);
+  });
+
+  it('expands on activation and says so', async () => {
+    const el = await fixture('config-section', { heading: 'Theme' });
+
+    shadow<HTMLButtonElement>(el, '.header')?.click();
+    await update(el, {});
+
+    expect(shadow(el, '.header')?.getAttribute('aria-expanded')).toBe('true');
+    expect((shadow(el, '.body') as HTMLElement).hidden).toBe(false);
+  });
+
+  it('starts expanded when the host asks it to', async () => {
+    const el = await fixture('config-section', { heading: 'Libraries', open: true });
+
+    expect(shadow(el, '.header')?.getAttribute('aria-expanded')).toBe('true');
+  });
+});
+
+describe('<downloads-view> tabs', () => {
+  it('is a tablist of tabs owning a panel', async () => {
+    const el = await fixture('downloads-view');
+
+    const tabs = shadowAll<HTMLButtonElement>(el, '[role="tab"]');
+    const panel = shadow(el, '[role="tabpanel"]');
+
+    expect(shadow(el, '[role="tablist"]')).toBeTruthy();
+    expect(tabs).toHaveLength(2);
+    expect(tabs.map((t) => t.getAttribute('aria-selected'))).toEqual([
+      'true',
+      'false',
+    ]);
+    expect(tabs[0]!.getAttribute('aria-controls')).toBe(panel?.id);
+  });
+
+  it('carries a roving tab stop, not two', async () => {
+    const el = await fixture('downloads-view');
+
+    const tabs = shadowAll<HTMLButtonElement>(el, '[role="tab"]');
+
+    expect(tabs.map((t) => t.tabIndex)).toEqual([0, -1]);
+  });
+
+  it('moves and activates on ArrowRight, wrapping', async () => {
+    const el = await fixture('downloads-view');
+
+    const tablist = shadow<HTMLElement>(el, '[role="tablist"]')!;
+    tablist.dispatchEvent(
+      new KeyboardEvent('keydown', { key: 'ArrowRight', bubbles: true }),
+    );
+    await update(el, {});
+
+    expect(
+      shadowAll(el, '[role="tab"]').map((t) => t.getAttribute('aria-selected')),
+    ).toEqual(['false', 'true']);
+    expect(shadow(el, '[role="tabpanel"]')?.id).toBe('panel-downloads');
+
+    tablist.dispatchEvent(
+      new KeyboardEvent('keydown', { key: 'ArrowRight', bubbles: true }),
+    );
+    await update(el, {});
+
+    expect(shadow(el, '[role="tabpanel"]')?.id).toBe('panel-requests');
+  });
+});