fix(shell): publish the active view, so both navs follow the back path

The nav components learned where the user was from the `navigate`
CustomEvent, which only the outbound path dispatches: `popstate` calls
`handleNavigate()` directly. So a back-navigation left both of them
highlighting the view just left — desktop included, at any width, on
any back across two primary views. Opening a detail view was the same
cause wearing a different symptom: `app-sidebar` guarded on its own
item list and kept its highlight, `bottom-nav` did not and lit nothing.

It cannot be fixed by re-dispatching `navigate` — `index.ts` is that
event's document listener, so that is an infinite loop, and "please go
to X" is not the statement being made. `activeViewStore` is the shell
saying "the active view is now X", once per navigation, `popstate`
included; both navs read it through a controller and hold no
`activeView` of their own.

A store rather than an event because a component that mounts *after* a
navigation still has to know: `bottom-nav`'s drawer builds its
`app-sidebar` on open, and that copy had heard nothing at all, so the
drawer opened on Home from any page in the app.

Closes #72

frontend/test/components/bottom-nav.test.ts

@@ -3,15 +3,21 @@
  *
  * Three of these are about the thing that makes a second nav dangerous:
  * it has to agree with the first one. `bottom-nav` emits the same
- * bubbling, composed `navigate` event `app-sidebar` does and listens
- * for that event globally, so a navigation from anywhere — a card, a
- * detail view, the drawer's own sidebar — moves its highlight too. A
- * tab bar that only tracks its own clicks looks right until the moment
- * the user arrives somewhere by another route.
+ * bubbling, composed `navigate` event `app-sidebar` does, and reads
+ * which tab is lit from `activeViewStore` — the shell's one statement
+ * of where the user is — so it follows a navigation from anywhere: a
+ * card, a detail view, the drawer's own sidebar, or the back gesture.
+ *
+ * That last one is why the source is the store and not the `navigate`
+ * event these tests used to dispatch. `popstate` dispatches no
+ * `navigate` (index.ts calls `handleNavigate` directly), so a tab bar
+ * listening for the event looked right until the user pressed back —
+ * #72.
  */
 import { describe, expect, it, beforeEach } from 'vitest';
 
 import '@components/bottom-nav/bottom-nav';
+import { activeViewStore } from '@store/active-view-store';
 import type { BottomNav } from '@components/bottom-nav/bottom-nav';
 import { fixture, shadow, shadowAll, update } from '@test/support/render';
 import { resetHarness } from '@test/support/harness';
@@ -21,6 +27,12 @@ type Nav = BottomNav;
 const tabs = (el: HTMLElement) =>
   shadowAll<HTMLButtonElement>(el, 'nav button');
 
+/** The testids of whatever the bar says is the current page. */
+const current = (el: HTMLElement) =>
+  tabs(el)
+    .filter((b) => b.getAttribute('aria-current') === 'page')
+    .map((b) => b.dataset.testid);
+
 /** Resolve on one occurrence of an event, or reject loudly on time. */
 const once = (el: Element, name: string, timeoutMs = 2000) =>
   new Promise<void>((resolve, reject) => {
@@ -70,36 +82,55 @@ describe('bottom-nav', () => {
   it('follows a navigation it did not send', async () => {
     const el = await fixture<Nav>('bottom-nav');
 
-    document.dispatchEvent(new CustomEvent('navigate', {
-      detail: { view: 'tracks' },
-      bubbles: true,
-      composed: true,
-    }));
+    activeViewStore.setView('tracks', true);
     await update(el, {});
 
-    const current = tabs(el)
-      .filter((b) => b.getAttribute('aria-current') === 'page')
-      .map((b) => b.dataset.testid);
-
-    expect(current).toEqual(['tab-tracks']);
+    expect(current(el)).toEqual(['tab-tracks']);
   });
 
   it('marks exactly one tab current, and none for a view it has no tab for', async () => {
     const el = await fixture<Nav>('bottom-nav');
 
-    document.dispatchEvent(new CustomEvent('navigate', {
-      detail: { view: 'settings' },
-      bubbles: true,
-      composed: true,
-    }));
+    activeViewStore.setView('settings', true);
     await update(el, {});
 
     // Settings lives in the drawer, so nothing in the bar is current.
     // Leaving Home highlighted would be a tab bar lying about where
     // the user is.
-    expect(
-      tabs(el).filter((b) => b.getAttribute('aria-current') === 'page'),
-    ).toHaveLength(0);
+    expect(current(el)).toEqual([]);
+  });
+
+  it('keeps the parent tab lit while a detail view is open', async () => {
+    const el = await fixture<Nav>('bottom-nav');
+
+    activeViewStore.setView('albums', true);
+    // A detail view reports itself and is not primary, so it changes
+    // nothing. This is the first half of #72: the bar used to take the
+    // name, match it against no tab, and light nothing at all — while
+    // `app-sidebar`, which guarded on its own item list, kept the
+    // highlight. Neither was deliberate and the two disagreed.
+    activeViewStore.setView('explore-album-details', false);
+    await update(el, {});
+
+    expect(current(el)).toEqual(['tab-albums']);
+  });
+
+  it('follows the back path, which dispatches no navigate event', async () => {
+    const el = await fixture<Nav>('bottom-nav');
+
+    activeViewStore.setView('albums', true);
+    activeViewStore.setView('tracks', true);
+    await update(el, {});
+    expect(current(el)).toEqual(['tab-tracks']);
+
+    // What `popstate` does: the shell replays the entry through
+    // `handleNavigate` without dispatching `navigate`. A bar listening
+    // for the event stayed on Tracks — the view just left, confidently
+    // wrong rather than merely blank.
+    activeViewStore.setView('albums', true);
+    await update(el, {});
+
+    expect(current(el)).toEqual(['tab-albums']);
   });
 
   it('closes the drawer when a navigation happens', async () => {

frontend/test/components/chrome.test.ts

@@ -10,6 +10,7 @@ import '@components/sidebar/app-sidebar';
 import '@components/library-filter/library-filter';
 import '@components/library-status-indicator/library-status-indicator';
 import { Events } from '../../src/events';
+import { activeViewStore } from '@store/active-view-store';
 import { emit, stub, flush, calls, lastArgs } from '@test/support/harness';
 import {
   fixture,
@@ -49,6 +50,8 @@ describe('<app-sidebar>', () => {
   });
 
   it('marks exactly one item as the current page', async () => {
+    activeViewStore.setView('home', true);
+
     const el = await fixture('app-sidebar');
 
     const current = shadowAll(el, 'li button').filter(
@@ -72,17 +75,49 @@ describe('<app-sidebar>', () => {
     expect(seen).toEqual(['artists']);
   });
 
-  it('moves aria-current to the clicked destination', async () => {
+  it('moves aria-current with the shell, not with the click', async () => {
+    activeViewStore.setView('home', true);
+
     const el = await fixture('app-sidebar');
 
     shadow<HTMLElement>(el, '[data-testid="nav-genres"]')?.click();
     await el.updateComplete;
 
+    // The click asks; it does not answer. The sidebar used to move its
+    // own highlight optimistically, which is the second opinion #72
+    // removed -- one component deciding where the user is, while the
+    // shell decided separately and `bottom-nav` decided a third way.
+    expect(
+      shadow(el, '[data-testid="nav-genres"]')?.getAttribute('aria-current'),
+    ).toBe('false');
+
+    // What the shell does with that event, in one line.
+    activeViewStore.setView('genres', true);
+    await update(el, {});
+
     expect(
       shadow(el, '[data-testid="nav-genres"]')?.getAttribute('aria-current'),
     ).toBe('page');
   });
 
+  it('follows the back path, which dispatches no navigate event', async () => {
+    activeViewStore.setView('albums', true);
+
+    const el = await fixture('app-sidebar');
+
+    // `popstate` replays an entry through `handleNavigate` directly, so
+    // there is no `navigate` event to hear -- which is why the sidebar
+    // stayed on the view the user had just left (#72).
+    activeViewStore.setView('tracks', true);
+    await update(el, {});
+
+    expect(
+      shadowAll(el, 'li button')
+        .filter((item) => item.getAttribute('aria-current') === 'page')
+        .map((item) => item.getAttribute('data-testid')),
+    ).toEqual(['nav-tracks']);
+  });
+
   it('looks the way it did last time', async () => {
     const el = await fixture('app-sidebar');
 

frontend/test/stores/view-stores.test.ts

@@ -1,11 +1,13 @@
 /**
- * The three small stores behind view chrome: the global search term,
- * the track list's column set, and the explore cache that keeps detail
- * pages from re-fetching what a search already returned.
+ * The small stores behind view chrome: the global search term, the
+ * active view both navs highlight, the track list's column set, and
+ * the explore cache that keeps detail pages from re-fetching what a
+ * search already returned.
  */
 import { describe, expect, it, beforeEach } from 'vitest';
 
 import { searchStore } from '@store/search-store';
+import { activeViewStore } from '@store/active-view-store';
 import { trackListStore } from '@store/tracklist-store';
 import { exploreCache, ARTIST_IMAGE_CACHE_LIMIT } from '@store/explore-cache';
 import { Events } from '../../src/events';
@@ -80,6 +82,57 @@ describe('search store', () => {
   });
 });
 
+describe('active view store', () => {
+  beforeEach(() => {
+    activeViewStore.setView('home', true);
+  });
+
+  it('holds the primary view the shell navigated to', () => {
+    activeViewStore.setView('albums', true);
+
+    expect(activeViewStore.get()).toBe('albums');
+    expect(activeViewStore.isActive('albums')).toBe(true);
+    expect(activeViewStore.isActive('tracks')).toBe(false);
+  });
+
+  it('leaves the primary view lit while a detail view is open', () => {
+    activeViewStore.setView('albums', true);
+    activeViewStore.setView('explore-album-details', false);
+
+    // #72's third finding, made deliberate: a detail view is not a
+    // destination in either nav, and the tab it was opened from is
+    // where the user still is. `app-sidebar` did this by accident (it
+    // guarded on its own item list) and `bottom-nav` did not do it at
+    // all, which is why one looked right and the other looked broken.
+    expect(activeViewStore.get()).toBe('albums');
+  });
+
+  it('does not notify when the view is unchanged', () => {
+    let notifications = 0;
+    const off = activeViewStore.subscribe(() => {
+      notifications += 1;
+    });
+
+    activeViewStore.setView('albums', true);
+    activeViewStore.setView('albums', true);
+    activeViewStore.setView('explore-album-details', false);
+    off();
+
+    expect(notifications).toBe(1);
+  });
+
+  it('lights nothing for a view with no name', () => {
+    // The store starts empty rather than defaulting to a view, because
+    // a written-down default is right only while `GetDefaultPage()`
+    // agrees with it. That is only safe if the empty value matches
+    // nothing: `isActive` compares strings, and a component asking
+    // about an id it does not have must not light up.
+    activeViewStore.setView('', true);
+
+    expect(activeViewStore.isActive('')).toBe(false);
+  });
+});
+
 describe('track list store', () => {
   it('starts from the default column set', () => {
     expect(trackListStore.getState().columnIds.length).toBeGreaterThan(0);