fix(ios): reduce GPU memory pressure to prevent WebKit GPU process crash (#3842)

On iOS, navigating to a book group in the library caused the WebKit GPU
process to exceed its 300 MB jetsam limit (peaking at ~328 MB), resulting
in a blank screen flash and broken scroll state.

Three changes reduce peak GPU memory usage:

- Add overscan={200} to VirtuosoGrid/Virtuoso so only items within 200px
  of the viewport are rendered, limiting simultaneous image decoding
- Add loading="lazy" to both Image components in BookCover so the browser
  defers decoding offscreen cover images
- Conditionally mount the <video> and <audio> elements in AtmosphereOverlay
  only when atmosphere mode is active, eliminating idle H.264 decoder
  memory overhead

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

apps/readest-app/src/__tests__/components/AtmosphereOverlay.test.tsx

@@ -0,0 +1,41 @@
+import { describe, it, expect, vi, afterEach } from 'vitest';
+import { render, cleanup } from '@testing-library/react';
+
+// Mock zustand stores before importing the component
+vi.mock('@/store/atmosphereStore', () => ({
+  useAtmosphereStore: vi.fn(),
+}));
+vi.mock('@/store/themeStore', () => ({
+  useThemeStore: vi.fn(),
+}));
+
+import AtmosphereOverlay from '@/components/AtmosphereOverlay';
+import { useAtmosphereStore } from '@/store/atmosphereStore';
+import { useThemeStore } from '@/store/themeStore';
+
+afterEach(cleanup);
+
+const setupStores = (active: boolean) => {
+  vi.mocked(useAtmosphereStore).mockImplementation((selector: unknown) =>
+    (selector as (s: { active: boolean }) => unknown)({ active }),
+  );
+  vi.mocked(useThemeStore).mockImplementation((selector: unknown) =>
+    (selector as (s: { isDarkMode: boolean }) => unknown)({ isDarkMode: false }),
+  );
+};
+
+describe('AtmosphereOverlay', () => {
+  it('does not render <video> element when inactive', () => {
+    setupStores(false);
+    const { container } = render(<AtmosphereOverlay />);
+    const video = container.querySelector('video');
+    expect(video).toBeNull();
+  });
+
+  it('renders <video> element when active', () => {
+    setupStores(true);
+    const { container } = render(<AtmosphereOverlay />);
+    const video = container.querySelector('video');
+    expect(video).toBeTruthy();
+  });
+});

apps/readest-app/src/__tests__/components/BookCover.test.tsx

@@ -0,0 +1,41 @@
+import { describe, it, expect, vi, afterEach } from 'vitest';
+import { render, cleanup } from '@testing-library/react';
+
+import BookCover from '@/components/BookCover';
+import { Book } from '@/types/book';
+
+vi.mock('next/image', () => ({
+  __esModule: true,
+  default: (props: Record<string, unknown>) => {
+    // biome-ignore lint/a11y/useAltText: test mock; alt comes from spread props
+    return <img {...props} />;
+  },
+}));
+
+afterEach(cleanup);
+
+const makeBook = (overrides?: Partial<Book>): Book =>
+  ({
+    hash: 'abc123',
+    title: 'Test Book',
+    author: 'Test Author',
+    format: 'epub',
+    coverImageUrl: 'https://example.com/cover.jpg',
+    ...overrides,
+  }) as Book;
+
+describe('BookCover', () => {
+  it('passes loading="lazy" to crop-mode Image', () => {
+    const { container } = render(<BookCover book={makeBook()} coverFit='crop' />);
+    const img = container.querySelector('img.cover-image');
+    expect(img).toBeTruthy();
+    expect(img?.getAttribute('loading')).toBe('lazy');
+  });
+
+  it('passes loading="lazy" to fit-mode Image', () => {
+    const { container } = render(<BookCover book={makeBook()} coverFit='fit' />);
+    const img = container.querySelector('img.cover-image');
+    expect(img).toBeTruthy();
+    expect(img?.getAttribute('loading')).toBe('lazy');
+  });
+});

apps/readest-app/src/app/library/components/Bookshelf.tsx

@@ -591,6 +591,7 @@ const Bookshelf: React.FC<BookshelfProps> = ({
       {scrollParentEl && hasItems && isGridMode && (
         <VirtuosoGrid<unknown, BookshelfListContext>
           customScrollParent={scrollParentEl}
+          overscan={200}
           totalCount={gridTotalCount}
           components={GRID_VIRTUOSO_COMPONENTS}
           context={listContext}
@@ -601,6 +602,7 @@ const Bookshelf: React.FC<BookshelfProps> = ({
       {scrollParentEl && hasItems && !isGridMode && (
         <Virtuoso
           customScrollParent={scrollParentEl}
+          overscan={200}
           totalCount={sortedBookshelfItems.length}
           components={LIST_VIRTUOSO_COMPONENTS}
           computeItemKey={computeItemKey}

apps/readest-app/src/components/AtmosphereOverlay.tsx

@@ -16,20 +16,15 @@ const AtmosphereOverlay = () => {
   useEffect(() => {
     const video = videoRef.current;
     const audio = audioRef.current;
-    if (!video || !audio) return;
 
     if (active) {
       document.body.classList.add('atmosphere');
-      video.play().catch(() => {});
+      video?.play()?.catch(() => {});
       if (!isInitialMount.current) {
-        audio.play().catch(() => {});
+        audio?.play()?.catch(() => {});
       }
     } else {
       document.body.classList.remove('atmosphere');
-      video.pause();
-      video.currentTime = 0;
-      audio.pause();
-      audio.currentTime = 0;
     }
     isInitialMount.current = false;
   }, [active]);
@@ -38,23 +33,27 @@ const AtmosphereOverlay = () => {
     const audio = audioRef.current;
     if (!audio || !active) return;
     audio.src = audioSrc;
-    audio.play().catch(() => {});
+    audio.play()?.catch(() => {});
     // eslint-disable-next-line react-hooks/exhaustive-deps
   }, [audioSrc]);
 
   return (
     <>
-      <video
-        ref={videoRef}
-        id='atmosphere-overlay'
-        src='/assets/komorebi.mp4'
-        loop
-        muted
-        playsInline
-        preload='none'
-      />
-      {/* biome-ignore lint/a11y/useMediaCaption: ambient background audio, no spoken content */}
-      <audio ref={audioRef} id='forest-audio' src={audioSrc} loop preload='none' />
+      {active && (
+        <video
+          ref={videoRef}
+          id='atmosphere-overlay'
+          src='/assets/komorebi.mp4'
+          loop
+          muted
+          playsInline
+          preload='none'
+        />
+      )}
+      {active && (
+        // biome-ignore lint/a11y/useMediaCaption: ambient background audio, no spoken content
+        <audio ref={audioRef} id='forest-audio' src={audioSrc} loop preload='none' />
+      )}
     </>
   );
 };

apps/readest-app/src/components/BookCover.tsx

@@ -74,6 +74,7 @@ const BookCover: React.FC<BookCoverProps> = memo<BookCoverProps>(
               src={book.metadata?.coverImageUrl || book.coverImageUrl!}
               alt={book.title}
               fill={true}
+              loading='lazy'
               className={clsx('cover-image crop-cover-img object-cover', imageClassName)}
               onLoad={handleImageLoad}
               onError={handleImageError}
@@ -96,6 +97,7 @@ const BookCover: React.FC<BookCoverProps> = memo<BookCoverProps>(
                 width={0}
                 height={0}
                 sizes='100vw'
+                loading='lazy'
                 className={clsx(
                   'cover-image fit-cover-img h-auto max-h-full w-auto max-w-full shadow-md',
                   imageClassName,