fix: speed up alternate-screen history reads

[ogulcancelik]

Summary

• advance alternate-screen history traversal from parsed PTY output instead of paying a fixed 120ms delay after every wheel batch
• coalesce fragmented redraws, honor synchronized-output boundaries, and keep traversal/restore deadlines bounded
• preserve conservative wheel batches across Pi, Claude Code, and OpenCode while restoring the original viewport

Validation

• just check
• Pi/Luna, 500 rows: 4.42s → 2.73s with contiguous output
• Claude Haiku, 300 rows: 0.57s across five repeated reads with contiguous output
• OpenCode, 300 rows: 0.63s across five repeated reads with contiguous output

refs #2387

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Review details

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro Plus

Run ID: d24c91c3-22b1-402b-aa02-ae41804228ab

📥 Commits

Reviewing files that changed from the base of the PR and between 3825c0c and 39e52d8.

📒 Files selected for processing (4)

• src/pane.rs
• src/server/alt_screen_read.rs
• src/server/headless.rs
• src/terminal/runtime.rs

📝 Walkthrough

Walkthrough

The change adds PTY content-sequence tracking, sequence-stable terminal snapshots, and deadline-based alternate-screen traversal. Alternate-screen reads now coordinate output quiet periods, synchronized redraws, harvesting, and restoration with expanded test coverage.

Changes

Alternate-screen read synchronization

Layer / File(s)	Summary
Track PTY content updates src/pane.rs	PaneRuntime tracks content updates around PTY processing. Runtime construction and test fixtures initialize the counter.
Capture stable snapshots src/terminal/runtime.rs, src/server/headless.rs	TerminalRuntime returns snapshots only when the content sequence is even and unchanged. Headless read specifications propagate the sequence.
Coordinate traversal and restoration src/server/alt_screen_read.rs	Pending reads use quiet periods, redraw tracking, synchronized output handling, per-step deadlines, and sequence-validated snapshots. Tests cover traversal deadlines, redraw coalescing, harvesting, and restoration retries.

Estimated code review effort: 4 (Complex) | ~60 minutes

Sequence Diagram(s)

sequenceDiagram
  participant PTY
  participant PaneRuntime
  participant TerminalRuntime
  participant PendingAltScreenRead
  PTY->>PaneRuntime: process terminal bytes
  PaneRuntime-->>TerminalRuntime: expose content_seq
  TerminalRuntime-->>PendingAltScreenRead: return stable snapshot and sequence
  PendingAltScreenRead->>PendingAltScreenRead: track output and deadlines
  PendingAltScreenRead->>PTY: issue traversal or restore step
  PTY-->>PendingAltScreenRead: produce redraw output

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🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly summarizes the main change: improving the speed of alternate-screen history reads.
Description check	✅ Passed	The description directly explains the traversal, redraw, deadline, viewport, and validation changes in the pull request.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
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.

✨ 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 fix/fast-alt-screen-read

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Actionable comments posted: 1

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro Plus

Run ID: b0ac5f89-a3cc-426c-8452-6fd3cda3dc97

📥 Commits

Reviewing files that changed from the base of the PR and between 3825c0c and 660e2e2.

📒 Files selected for processing (4)

• src/pane.rs
• src/server/alt_screen_read.rs
• src/server/headless.rs
• src/terminal/runtime.rs

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Note

GitHub couldn't provide a complete incremental comparison for this pull request, so CodeRabbit is performing a full review instead. This review may take a little longer.

[greptile-apps]

Greptile Summary

The PR replaces fixed alternate-screen polling delays with PTY-output-driven traversal while preserving bounded traversal and viewport restoration.

• Adds an atomic content sequence around PTY processing so snapshots can detect concurrent updates.
• Coalesces fragmented redraws and observes synchronized-output boundaries.
• Adds traversal, restoration, redraw, and timeout regression tests.
• Passes the initial snapshot sequence from the headless server into each pending read.

Confidence Score: 4/5

The PR should not merge until the outstanding alternate-screen deadline bug is fixed because delayed output can make the headless event loop busy-poll.

Delayed PTY output can establish a quiet window while the separate wheel-step deadline is already expired, and the pending read's deadline feeds the single-threaded headless timer loop directly, leaving the previously reported busy-poll path unresolved.

Files Needing Attention: src/server/alt_screen_read.rs, src/server/headless.rs

Important Files Changed

Filename	Overview
src/server/alt_screen_read.rs	Reworks traversal into output-driven, deadline-bounded phases and adds extensive tests; the previously reported deadline-scheduling issue remains outstanding.
src/pane.rs	Adds an atomic sequence counter bracketing terminal processing for stable snapshot detection.
src/terminal/runtime.rs	Adds a bounded retry helper that returns a snapshot only when the content sequence remains stable.
src/server/headless.rs	Captures the initial snapshot sequence, supplies it to pending reads, and schedules their reported deadlines in the main loop.

Sequence Diagram

sequenceDiagram
  participant API as Pane read request
  participant H as Headless server loop
  participant R as PendingAltScreenRead
  participant T as TerminalRuntime
  participant P as PTY application

  API->>H: request alternate-screen history
  H->>T: snapshot_with_seq()
  H->>R: start(snapshot, content_seq)
  R->>T: send wheel batch
  P-->>T: process PTY redraw bytes
  T->>T: increment content_seq before/after parsing
  H->>R: poll(now)
  R->>T: read content_seq and stable snapshot
  alt synchronized output active
    R-->>H: wait for frame boundary
  else output still fragmented
    R-->>H: wait for quiet deadline
  else stable redraw
    R->>R: merge history or restore viewport
  end

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