[chat-perf] layoutDurationMs regressed ~20% in thinking-response scenario

[vs-code-engineering]

Summary:
New regression detected: layoutDurationMs in the thinking-response scenario increased by +20.5% (from 130.21ms ±4ms to 156.95ms ±5.43ms).

Evidence:

• Current: 156.95ms ±5.43ms
• Baseline: 130.21ms ±4ms
• Absolute delta: +26.74ms
• Relative delta: +20.5%
• p-value: 0 (statistically significant)
• No prior baseline runs available (first run), but the change is large and outside typical noise for this metric.
• No prior Slack threads or open issues cover this regression.

Suspect commits:

• d92b683 (Merge branch 'main' into fix-281241): includes changes to chat widget, layout, and chatContentParts/chatThinkingContentPart.ts, which directly affect layout timing.

Suggested action:

• Investigate recent changes in chat widget and layout logic, especially in chatContentParts/chatThinkingContentPart.ts and related files from d92b683.
• Confirm if the regression is expected or needs mitigation.
• Monitor for further layout regressions in chat scenarios.