fix(home): gate the card play button on the device having hover

The play button on a home shelf's cover cards is revealed by :hover, and
a touch long-press synthesises a hover state in the WebView — so on a
phone it flashed into view during the 500ms hold that
utils/long-press.ts is measuring for a context menu. A control appearing
because the user was reaching for a different one.

It is gated on `(hover: hover) and (pointer: fine)` rather than on width,
so it is absent on any touch device and present on a desktop with a small
window. A phone user taps the album and plays from the detail view, so
nothing replaces it.

The default outside the query is display:none, not opacity:0. An
opacity-0 button still takes taps and is still in the accessibility tree,
so leaving the reveal as the only guarded part would keep the hit area
for a control the phone can never show.

The test asserts the parsed stylesheet rather than rendering as a phone,
and says so: CDP's Emulation.setEmulatedMedia does not reach this tier's
iframe, so matchMedia still answers `hover: hover` after it is set. The
regression worth catching is someone hoisting the rule back out of the
query as a tidy-up — a change no desktop assertion can see.

Closes #68

e2e/specs/album-dropdown.spec.ts

@@ -1,6 +1,12 @@
 import { test, expect } from '../support/fixtures.js';
 import type { Page } from '@playwright/test';
 
+/**
+ * How far the scroll test scrolls. One constant, because the guard and
+ * the assertion have to agree about it — they did not, which is #133.
+ */
+const SCROLL_TARGET = 80;
+
 /**
  * Plan 007 phase 5: expanding an album shows its tracks.
  *
@@ -104,20 +110,27 @@ test.describe('the album dropdown', () => {
     await app.setViewportSize({ width: 900, height: 600 });
 
     try {
-      await expect.poll(() => scrollRange(app)).toMatchObject({
-        scrollable: true,
-        overflowY: 'auto',
-      });
+      // Wait for the range the assertion below actually needs, not for
+      // "scrollable at all" (#133). The guard used to be
+      // `scrollHeight > clientHeight + 40` while the next line asks to
+      // reach 80, so any range in 41-79 satisfied it and could not
+      // satisfy the assertion — and the grid passes through exactly
+      // that while it settles, because it recomputes its columns after
+      // the resize rather than during it. The settled range here is
+      // 330, so this waits rather than weakening anything.
+      await expect
+        .poll(() => scrollRange(app))
+        .toMatchObject({ room: true, overflowY: 'auto' });
 
-      await app.evaluate(() => {
+      await app.evaluate((target) => {
         const sc = document
           .querySelector('cover-grid')
           ?.shadowRoot?.querySelector('.grid-scroll-container');
 
-        if (sc) sc.scrollTop = 80;
-      });
+        if (sc) sc.scrollTop = target;
+      }, SCROLL_TARGET);
 
-      expect(await scrollTop(app)).toBe(80);
+      expect(await scrollTop(app)).toBe(SCROLL_TARGET);
 
       // And the dropdown it opens is on screen, wherever the manager
       // decides that leaves the scroll. It is *not* "the position is
@@ -250,16 +263,19 @@ async function closeDropdown(app: Page): Promise<void> {
 
 /** Whether the grid can scroll at all, which decides if a probe can move. */
 async function scrollRange(app: Page) {
-  return app.evaluate(() => {
+  return app.evaluate((target) => {
     const sc = document
       .querySelector('cover-grid')
       ?.shadowRoot?.querySelector('.grid-scroll-container');
 
     return {
-      scrollable: !!sc && sc.scrollHeight > sc.clientHeight + 40,
+      // `room` is the precondition of the assertion that follows it:
+      // enough range to actually reach the target. A threshold below
+      // what the caller depends on is not a guard.
+      room: !!sc && sc.scrollHeight - sc.clientHeight >= target,
       overflowY: sc ? getComputedStyle(sc).overflowY : '',
     };
-  });
+  }, SCROLL_TARGET);
 }
 
 async function scrollTop(app: Page): Promise<number> {

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

@@ -160,29 +160,52 @@ export class HomeView extends ViewLifecycleMixin(LitElement) {
                 user-select: none;
             }
 
+            /*
+             * The hover play button is a *hover* affordance, so it is
+             * gated on the device having hover rather than on width.  A
+             * touch long-press synthesises a hover state in the WebView,
+             * so on a phone it flashed into view during the 500ms hold
+             * that utils/long-press.ts is measuring for a context menu —
+             * a control appearing because you were reaching for a
+             * different one.  A phone user taps the album and plays from
+             * the detail view, so there is nothing to replace it with.
+             *
+             * display:none outside the query rather than opacity:0 on
+             * its own: an opacity-0 button still takes taps and is
+             * still in the accessibility tree, so the invisible control
+             * would keep the hit area it was never meant to have on
+             * touch. Everything else stays inside, so the desktop
+             * animation is unchanged.
+             */
             .play {
-                position: absolute;
-                right: 8px;
-                bottom: 8px;
-                width: 38px;
-                height: 38px;
-                border: none;
-                border-radius: 50%;
-                background: var(--yj-accent, #ffd43b);
-                color: var(--yj-accent-fg, #000);
-                display: flex;
-                align-items: center;
-                justify-content: center;
-                cursor: pointer;
-                opacity: 0;
-                transform: translateY(6px);
-                transition: opacity 0.12s ease, transform 0.12s ease;
+                display: none;
             }
 
-            .card:hover .play,
-            .card:focus-within .play {
-                opacity: 1;
-                transform: translateY(0);
+            @media (hover: hover) and (pointer: fine) {
+                .play {
+                    position: absolute;
+                    right: 8px;
+                    bottom: 8px;
+                    width: 38px;
+                    height: 38px;
+                    border: none;
+                    border-radius: 50%;
+                    background: var(--yj-accent, #ffd43b);
+                    color: var(--yj-accent-fg, #000);
+                    display: flex;
+                    align-items: center;
+                    justify-content: center;
+                    cursor: pointer;
+                    opacity: 0;
+                    transform: translateY(6px);
+                    transition: opacity 0.12s ease, transform 0.12s ease;
+                }
+
+                .card:hover .play,
+                .card:focus-within .play {
+                    opacity: 1;
+                    transform: translateY(0);
+                }
             }
 
             .name {

frontend/test/components/hover-affordance.test.ts

@@ -0,0 +1,85 @@
+/**
+ * A hover affordance is gated on the device having hover.
+ *
+ * The home page's cover cards reveal a play button on :hover. A touch
+ * long-press synthesises a hover state in the WebView, so on a phone
+ * that button flashed into view during the 500ms hold that
+ * utils/long-press.ts is measuring for a context menu — a control
+ * appearing because the user was reaching for a different one.
+ *
+ * This is asserted against the *parsed stylesheet* rather than by
+ * emulating a touch device, and that is a limitation worth stating
+ * rather than hiding. CDP's Emulation.setEmulatedMedia does not reach
+ * this tier's iframe — matchMedia still answers `hover: hover` after it
+ * is set — so there is no way here to render the component as a phone
+ * would and read the computed style. What can be checked is the shape
+ * the browser actually built from the css`` literal: that the reveal
+ * lives inside a hover media query and that the default is display:none.
+ *
+ * Which is the regression worth catching anyway. The failure mode is
+ * someone hoisting the rule back out of the query for a one-line tidy —
+ * a change nothing renders differently on a desktop, so every other
+ * assertion in this repo passes and the phone silently regresses.
+ */
+import { describe, expect, it } from 'vitest';
+
+import '@components/home-view/home-view';
+import { fixture } from '@test/support/render';
+
+/** Every rule in the element's own adopted stylesheets, flattened. */
+function rulesOf(host: Element): { text: string; condition: string | null }[] {
+  const sheets = host.shadowRoot?.adoptedStyleSheets ?? [];
+  const out: { text: string; condition: string | null }[] = [];
+
+  for (const sheet of sheets) {
+    for (const rule of Array.from(sheet.cssRules)) {
+      if (rule instanceof CSSMediaRule) {
+        for (const inner of Array.from(rule.cssRules)) {
+          out.push({ text: inner.cssText, condition: rule.conditionText });
+        }
+
+        continue;
+      }
+
+      out.push({ text: rule.cssText, condition: null });
+    }
+  }
+
+  return out;
+}
+
+describe('the home card play button', () => {
+  it('reveals itself only where the device has hover', async () => {
+    const el = await fixture('home-view', {});
+    const rules = rulesOf(el);
+
+    // The sweep is worth nothing if it read no rules at all — the same
+    // first assertion icon-language.test.ts makes for the same reason.
+    expect(rules.length).toBeGreaterThan(0);
+
+    const reveals = rules.filter(
+      (r) => r.text.includes('.play') && /opacity:\s*1/.test(r.text),
+    );
+
+    expect(reveals.length).toBeGreaterThan(0);
+
+    for (const rule of reveals) {
+      expect(rule.condition).toMatch(/hover:\s*hover/);
+      expect(rule.condition).toMatch(/pointer:\s*fine/);
+    }
+  });
+
+  it('is display:none rather than transparent where it is absent', async () => {
+    const el = await fixture('home-view', {});
+
+    // opacity:0 alone would leave a button that still takes taps and is
+    // still in the accessibility tree, so a phone would keep the hit
+    // area for a control it can never see.
+    const unconditional = rulesOf(el).filter(
+      (r) => r.condition === null && r.text.startsWith('.play'),
+    );
+
+    expect(unconditional.length).toBeGreaterThan(0);
+    expect(unconditional.some((r) => /display:\s*none/.test(r.text))).toBe(true);
+  });
+});