perf(tui/chat): virtualize message rendering to remove O(n) reflows during streaming

[paulbettner]

Summary

• Motivation: Full-backlog reflows during streaming/shimmer caused UI jank in large histories. Render time scaled with total messages instead of visible rows.
• Approach: Virtualize chat message blocks and add a fast path so work scales with the viewport.

Changes

• Virtualized rendering:

  • Build per-block caches once in the slow path:
    • blockContents: rendered string per block
    • blockHeights: pre-measured line heights
    • blockPrefix: start line per block (includes leading blank + inter-block blanks)
  • Use sort.Search to find the first visible block and only synthesize visible rows.
  • Cache per-block line splits (blockLineCache) to avoid repeated strings.Split.

• Incremental indexing:

  • blockIndexByPartID maps part IDs to block indices (foundation for targeted per-part updates).
  • streamingReasoningID ensures shimmer applies only to the last in-flight reasoning block.

• Header optimization:

  • Cache the header; rebuild only when width changes (lastHeaderWidth, headerDirty).

• Shimmer gating:

  • Animate only when an assistant/tool is in-flight, re-render only when at bottom and backlog ≤ 2000 lines to avoid pathological repaint loops.

• Selection overlay correctness:

  • Disable the fast path when selection is active to preserve copy/highlighting fidelity.

Impact

• Streaming remains smooth on large histories; rendering work scales with the viewport, not the backlog.
• Lower CPU usage and fewer jank frames during token streaming and shimmer.

Non-goals

• No changes to message semantics, event ingestion, or UI look-and-feel.
• No new deps, flags, or config.

Testing

• Go TUI builds and runs (darwin/arm64 local).
• Manual checks:
  • Scrolling and streaming stay responsive with large histories.
  • Selection overlay still highlights accurately and copies cleanly.
  • Header stable; only recomputes on width changes.

Future Work (optional)

• Apply per-part incremental updates on MessagePartUpdated by adjusting only affected blocks and local prefix/height entries.
• Add a debug toggle to force-disable virtualization for repros.

Files

• packages/tui/internal/components/chat/messages.go

Notes for reviewers

• Fast path only engages when caches are warm and no selection overlay is active.
• Cache invalidation on resize/theme/session transitions is explicit to avoid stale layouts.

[rekram1-node]

We are rewriting the tui and it will ship this week, it should address all these performance issues