perf: window the transcript render to O(viewport)

[saucam]

Fixes the mouse-wheel scroll lag on large sessions.

Diagnosis

The transcript re-processed the whole buffer every frame. Even on a cache hit, ui/scrollback.rs did:

let lines = state.scrollback_build.lines.clone();        // clone ALL N lines, every frame
Paragraph::new(lines).wrap(Wrap{trim:false}).scroll((y,0)) // ratatui re-wraps ALL N lines, every frame

ratatui's Paragraph isn't virtualized — .wrap().scroll((y,0)) word-wraps the entire transcript and then throws away everything above row y to show ~40 rows. So a multi-thousand-line session re-wrapped the whole buffer on every mouse-wheel tick → the sluggishness. The per-message RenderCache helped, but this final step was still O(transcript) per frame.

Fix — O(viewport) per frame

• On each (rare) rebuild, also compute a prefix sum of per-line wrapped-row counts (ScrollbackBuild::row_offsets), using ratatui's own line_count per line so it stays byte-for-byte consistent with how the slice lays out.
• On render, visible_window() binary-searches the few logical lines intersecting [y, y+viewport) and hands ratatui only that slice (plus an intra-line scroll offset). No full clone, no full re-wrap.

Net: a 10k-line session now re-wraps ~viewport rows per scroll instead of 10k. Layout is identical — only the per-frame work changed.

Tests

visible_window is pure + unit-tested (top/middle/bottom windows, short-buffer, empty-safe, and a property test that the slice always covers the requested window). Full codeoid-tui suite: 162 passed. cargo fmt clean.

Note: this supersedes render/wrap.rs's rendered_rows/total_rendered_rows for scroll math (they were already unused on main); leaving them as a separate cleanup.

🤖 Generated with Claude Code

Summary by CodeRabbit

Release Notes

• Refactor
  • Improved scrollback scrolling performance with faster, windowed viewport rendering.
  • More accurate handling of wrapped text during vertical navigation for smoother transitions.
• Bug Fixes
  • Fixed scrollback viewport updates on scrolling so the visible content aligns correctly with the selected scroll position (including near the top and bottom of the buffer).

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

Plan: Pro Plus

Run ID: 47f0cae6-6cde-4c6f-a8c3-47178d257f05

📥 Commits

Reviewing files that changed from the base of the PR and between 0d37742 and f844da7.

📒 Files selected for processing (1)

• crates/codeoid-tui/src/ui/scrollback.rs

🚧 Files skipped from review as they are similar to previous changes (1)

• crates/codeoid-tui/src/ui/scrollback.rs

---

📝 Walkthrough

Walkthrough

The scrollback cache now stores wrapped-row prefix offsets, adds a helper to map viewport rows to logical-line slices, and updates rendering to build and use that slice instead of scrolling a paragraph over the full buffered transcript.

Changes

Scrollback viewport slicing

Layer / File(s)	Summary
Row offset state and window helper crates/codeoid-tui/src/state/scrollback_build.rs	ScrollbackBuild adds row_offsets, resets them in clear, and exposes visible_window(...) with tests for middle, top, bottom, oversized viewport, coverage, and empty-offset cases.
Windowed scrollback rendering crates/codeoid-tui/src/ui/scrollback.rs	The cache rebuild now computes per-line wrapped row counts and prefix offsets, and rendering uses visible_window(...) to render only lines[first..last] with an intra-line scroll offset. Tests validate cache-rebuild prefix sums and viewport scroll behavior.

Sequence Diagram(s)

sequenceDiagram
  participant Render
  participant ScrollbackBuild
  participant visible_window
  participant Paragraph

  Render->>ScrollbackBuild: compute per-line wrapped row counts
  Render->>ScrollbackBuild: store row_offsets and total_rendered_rows
  Render->>visible_window: visible_window(row_offsets, y, viewport_rows)
  visible_window-->>Render: first, intra, last
  Render->>Paragraph: build paragraph from lines[first..last]
  Render->>Paragraph: scroll((intra, 0))

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Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~25 minutes

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title 'perf: window the transcript render to O(viewport)' directly and precisely summarizes the main optimization: transforming transcript rendering from O(transcript length) to O(viewport) complexity by implementing windowed rendering.
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch perf/scrollback-windowed-render

---

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[codecov]

Codecov Report

❌ Patch coverage is 99.41176% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 41.86%. Comparing base (d7dea06) to head (f844da7).
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
crates/codeoid-tui/src/state/scrollback_build.rs	98.18%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main       #5      +/-   ##
==========================================
+ Coverage   36.79%   41.86%   +5.07%     
==========================================
  Files          30       30              
  Lines        5156     5317     +161     
==========================================
+ Hits         1897     2226     +329     
+ Misses       3259     3091     -168     

Flag	Coverage Δ
tui	41.86% <99.41%> (+5.07%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
crates/codeoid-tui/src/ui/scrollback.rs	66.75% <100.00%> (+66.75%)	⬆️
crates/codeoid-tui/src/state/scrollback_build.rs	80.00% <98.18%> (+80.00%)	⬆️

... and 1 file with indirect coverage changes