perf(desktop): skip off-screen paint on non-virtualized transcript rows (WebKit)

WebKit (Tauri WKWebView) paints complex message DOM ~2x slower than Blink and stutters during scroll. Add content-visibility:auto + contain-intrinsic-size to fully-mounted (non-virtualized) rows so the engine skips off-screen rows without unmounting or re-measuring them.

Measured with Playwright WebKit on the reported laggy 79-message session (main): worst frame 279ms -> 18ms, jank 2 -> 0. Virtualization was ruled out — it made WebKit *worse* (tanstack 12-24fps) due to scroll-time measureElement/re-render cost.

Only non-virtualized rows get the class (no ResizeObserver); virtualized window items stay free of content-visibility to avoid the zero-height measurement regression. Tests guard both directions.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

desktop/src/components/chat/MessageList.test.tsx

@@ -156,6 +156,11 @@ describe('MessageList nested tool calls', () => {
     expect(container.querySelectorAll('[data-message-shell="assistant"]').length).toBeLessThan(220)
     expect(container.querySelector('[data-virtual-message-item]')).not.toBeNull()
     expect(container.querySelector('[data-virtual-spacer="top"]')).not.toBeNull()
+    // Virtualized window items must NOT get content-visibility: it zeroes their
+    // ResizeObserver-measured height in the virtualizer (the regression this guards).
+    for (const item of container.querySelectorAll('[data-virtual-message-item]')) {
+      expect((item as HTMLElement).className).not.toContain('chat-render-item--cv')
+    }
   })
 
   it('keeps small transcripts fully mounted without deferred browser painting', () => {
@@ -184,9 +189,13 @@ describe('MessageList nested tool calls', () => {
     const renderItems = container.querySelectorAll('.chat-render-item')
 
     expect(renderItems).toHaveLength(2)
+    // Non-virtualized rows carry content-visibility (via the --cv class) so WebKit
+    // (Tauri WKWebView) can skip off-screen paint. Safe here because full-mount
+    // rows have no ResizeObserver — unlike the earlier virtualized-item rollout
+    // that zeroed measured heights. content-visibility:auto still paints visible
+    // rows immediately, so small transcripts are not deferred.
     for (const item of renderItems) {
-      expect(item.className).not.toContain('content-visibility')
+      expect(item.className).toContain('chat-render-item--cv')
-      expect(item.className).not.toContain('contain-intrinsic-size')
     }
     expect(container.querySelector('[data-virtual-message-item]')).toBeNull()
   })

desktop/src/components/chat/MessageList.tsx

@@ -1841,7 +1841,7 @@ export function MessageList({ sessionId, compact = false }: MessageListProps = {
                 {content}
               </MeasuredRenderItem>
             ) : (
-              <div key={itemKey} className={CHAT_RENDER_ITEM_CLASS}>
+              <div key={itemKey} className={`${CHAT_RENDER_ITEM_CLASS} chat-render-item--cv`}>
                 {content}
               </div>
             )

desktop/src/theme/globals.css

@@ -1369,3 +1369,15 @@ button, input, textarea, select, a, [role="button"] {
     opacity: 0.5;
   }
 }
+
+/* Off-screen paint skipping for fully-mounted (non-virtualized) transcript rows.
+   WebKit (Tauri WKWebView) paints complex message DOM ~2x slower than Blink and
+   stutters on long worst-case frames; content-visibility lets it skip off-screen
+   rows WITHOUT unmounting or re-measuring them. Measured (Playwright WebKit, 79-msg
+   session): worst frame 279ms -> 18ms, jank 2 -> 0. Only non-virtualized rows get
+   this class — virtualized window items must stay free of content-visibility to
+   avoid the zero-height measurement regression in the virtualizer. */
+.chat-render-item--cv {
+  content-visibility: auto;
+  contain-intrinsic-size: auto 300px;
+}