fix(ui): retire the blurhash on a timer, not transitionend

Under prefers-reduced-motion the img rule sets transition:none, so
toggling opacity fires no transitionend and the handler that unmounts
the blurhash never ran. The placeholder stayed mounted for the life of
the component — visible in the letterbox bars wherever the cover is
rendered with fit="contain", and a live canvas per tile everywhere else.

One duration constant now drives both the CSS transition and the timer,
so they cannot drift.

ui/src/common/CoverImage.jsx

@@ -7,6 +7,9 @@ import subsonic from '../subsonic'
 import { useImageUrl } from './useImageUrl'
 import { BlurHashCanvas } from './BlurHashCanvas'
 
+// Drives both the CSS transition and the timer that retires the blurhash, so they cannot drift.
+const fadeMs = 500
+
 const useStyles = makeStyles({
   // className supplies the size and shape; overflow:hidden clips the fills to a rounded shape.
   root: {
@@ -23,7 +26,7 @@ const useStyles = makeStyles({
   },
   img: {
     opacity: 0,
-    transition: 'opacity 500ms ease-out',
+    transition: `opacity ${fadeMs}ms ease-out`,
     '@media (prefers-reduced-motion: reduce)': { transition: 'none' },
   },
   imgVisible: { opacity: 1 },
@@ -60,6 +63,14 @@ export const CoverImage = ({
     setFaded(false)
   }, [url])
 
+  // Retire the blurhash on a timer rather than transitionend: under prefers-reduced-motion the
+  // transition is none, so the event never fires and the placeholder would stay up forever.
+  useEffect(() => {
+    if (!decoded || faded) return undefined
+    const timer = setTimeout(() => setFaded(true), fadeMs)
+    return () => clearTimeout(timer)
+  }, [decoded, faded])
+
   if (!record) return null
 
   const instant = cachedOnMount.current
@@ -90,7 +101,6 @@ export const CoverImage = ({
           style={{ objectFit: fit }}
           // Fading on decode, not on mount, keeps the image from ramping up before it can paint.
           onLoad={() => setDecoded(true)}
-          onTransitionEnd={() => setFaded(true)}
         />
       )}
     </div>

ui/src/common/CoverImage.test.jsx

@@ -1,4 +1,4 @@
-import { render, fireEvent } from '@testing-library/react'
+import { render, fireEvent, act } from '@testing-library/react'
 import { describe, it, expect, vi, beforeEach } from 'vitest'
 
 vi.mock('./useImageUrl', () => ({ useImageUrl: vi.fn() }))
@@ -54,21 +54,35 @@ describe('CoverImage', () => {
   })
 
   it('keeps the blurhash under the image until the fade ends', () => {
-    useImageUrl.mockReturnValue({ imgUrl: null, loading: true })
-    const { container, rerender } = render(<CoverImage record={withArt} />)
+    vi.useFakeTimers()
+    try {
+      useImageUrl.mockReturnValue({ imgUrl: null, loading: true })
+      const { container, rerender } = render(<CoverImage record={withArt} />)
 
-    // Blob arrives: the image mounts transparent, with the blurhash still behind it.
-    useImageUrl.mockReturnValue({ imgUrl: 'blob:abc', loading: false })
-    rerender(<CoverImage record={withArt} />)
-    const img = container.querySelector('img')
-    expect(img).not.toBeNull()
-    expect(container.querySelector('canvas')).not.toBeNull()
+      // Blob arrives: the image mounts transparent, with the blurhash still behind it.
+      useImageUrl.mockReturnValue({ imgUrl: 'blob:abc', loading: false })
+      rerender(<CoverImage record={withArt} />)
+      const img = container.querySelector('img')
+      expect(img).not.toBeNull()
+      expect(container.querySelector('canvas')).not.toBeNull()
 
-    // Decoding starts the cross-fade; the blurhash only goes away once it finishes.
-    fireEvent.load(img)
-    expect(container.querySelector('canvas')).not.toBeNull()
-    fireEvent.transitionEnd(img)
-    expect(container.querySelector('canvas')).toBeNull()
+      // Decoding starts the cross-fade; the blurhash only goes away once it finishes. The clock
+      // drives it, not transitionend, which never fires under prefers-reduced-motion.
+      act(() => {
+        fireEvent.load(img)
+      })
+      expect(container.querySelector('canvas')).not.toBeNull()
+      act(() => {
+        fireEvent.transitionEnd(img)
+      })
+      expect(container.querySelector('canvas')).not.toBeNull()
+      act(() => {
+        vi.advanceTimersByTime(500)
+      })
+      expect(container.querySelector('canvas')).toBeNull()
+    } finally {
+      vi.useRealTimers()
+    }
   })
 
   it('does not fade an image that was already cached on mount', () => {