Scroll performance via content-visibility

Problem/Motivation

On a fully expanded 200+ comment thread, scroll degrades to roughly 12-13 fps because the browser lays out the whole rendered comment tree on each scroll. ACT's own JavaScript accounts for under 1% of the cost; the bottleneck is layout. A measured A/B shows content-visibility: auto on comment rows yields roughly a 4x improvement with no functional change.

Steps to reproduce

• Expand a 200+ comment thread fully.
• Scroll; observe frame times degrade to ~12-13 fps and long layout tasks.

Proposed resolution

• Apply content-visibility: auto and an intrinsic size to comment rows.
• Verify in-page find, anchor (#comment-N) scrolling, and scrollbar stability; tune contain-intrinsic-size.

Remaining tasks

• Re-measure scroll frame times and confirm no scrollbar jump.

User interface changes

Smoother scrolling on large threads; no visual layout change.

API changes

None.

Data model changes

None.

AI-Generated: Yes (Used Opus 5 and DeepSeek Flash to review and discover these issues)