[chat-perf] layoutDurationMs regressed >20% in multiple chat scenarios (first run)

Summary: New regression detected: layoutDurationMs increased by 21–38% in text-only, tool-terminal, large-codeblock, file-links, thinking-response, and tool-read-file chat scenarios. This is the first run, so no prior baseline exists, but the magnitude and consistency across scenarios indicate a real regression.

Evidence:

• text-only: 126.34ms ±1.82ms → 166.63ms ±6.81ms (+31.9%)
• tool-terminal: 177.62ms ±3.32ms → 220.3ms ±9.02ms (+24.0%)
• large-codeblock: 167.31ms ±4.46ms → 216.57ms ±9.64ms (+29.4%)
• file-links: 211.82ms ±3.7ms → 261.71ms ±2.17ms (+23.6%)
• thinking-response: 154.67ms ±3.84ms → 212.82ms ±6.57ms (+37.6%)
• tool-read-file: 213.62ms ±7.18ms → 258.39ms ±4.69ms (+21.0%)

All p-values are 0, indicating high statistical significance.

Suspect commits:

• 3b1bd0b (justschen): "make sure layout accounts for agents window padding" — src/vs/workbench/contrib/chat/browser/chat.ts, src/vs/workbench/contrib/chat/browser/widget/chatListRenderer.ts, src/vs/workbench/contrib/chat/browser/widget/chatWidget.ts
• aaf5994 (justschen): "fix editor container overflow" — src/vs/workbench/contrib/chat/browser/widget/media/chat.css
• 33a3fa9 (justschen): "use plan widget instead of questions carousel when exiting plan mode" — src/vs/workbench/contrib/chat/browser/agentSessions/agentHost/agentHostSessionHandler.ts, src/vs/workbench/contrib/chat/browser/widget/chatListRenderer.ts

Suggested action:

• Investigate recent changes to chat widget rendering/layout logic, especially those affecting chatListRenderer, chatWidget, and chat.css.
• Roll back or optimize the affected code to restore layoutDurationMs to prior levels.
• Monitor subsequent nightly runs for confirmation and further trend analysis.