fix: preserve complete alternate-screen history reads

[ogulcancelik]

Summary

Fix incomplete alternate-screen history reads (herdr agent read --lines …). Pi's automatic scrollbar could limit the read to the current screen. Repeated wrapped rows and pinned headers could also leave sentences incomplete in Pi, Claude Code, and OpenCode.

• Ignore a recognized right-edge scrollbar only when comparing rows; preserve the original cells.
• Establish scroll distance from distinct matching rows instead of repeated continuation text.
• Refresh the leading overlap to recover text hidden by pinned headers, matching retained anchors by rendered text rather than wrap metadata.
• Reject repeated-only views and competing plausible scroll distances instead of guessing.

Validation

• TDD regressions for scrollbar appearance/movement, repeated continuation rows and larger scroll steps, pinned-header overlap, and equivalent text with different wrap metadata.
• Guard coverage for real edge text, ordinary borders, unrelated output, ambiguous repeated rows, competing distinct anchors, and chained reads after a scrollbar thumb moves.
• just check after rebasing onto current master and addressing both review findings: 3,399 tests passed.
• Live patched-binary smoke tests: Pi 0.85.1 (gpt-5.6-luna, default automatic scrollbar), Claude Code 2.1.268 (Haiku and Sonnet), and OpenCode 1.18.30 (MiMo V2.5 Free), at narrow (58–59 columns) and wide (86 columns) pane widths. Repeated after both review fixes. All eight runs recovered 100 complete sentences exactly once and in order, and restored the original visible text. Disposable test sessions and panes were cleaned up; the main session was untouched.

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ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Repository UI

Review profile: CHILL

Plan: Advanced

Run ID: cc93f730-e52f-475a-8943-63cad7a17da7

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Reviewing files that changed from the base of the PR and between c9f2f57 and 452ede2.

📒 Files selected for processing (1)

• src/terminal/history_read.rs

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📝 Walkthrough

Walkthrough

The terminal history merge now selects scroll distances from uniquely occurring row anchors. It normalizes scrollbar track and thumb characters during identity comparison when a scrollbar run is present. The merge refreshes overlapping rows and recovers text hidden by pinned headers. Tests cover repeated rows, pinned headers, appearing scrollbars, edge text, wrap metadata, and ambiguous alignments.

Priority: ⬇️ Low

Merge Risk: ⚪ Minimal · up to 452ed

The alternate-screen history merge changes are covered by regressions for anchors, pinned headers, scrollbars, wrapping, and ambiguous alignment; no current merge-blocking risk was identified.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 64.71% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 17 functions across 1 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly and concisely describes the primary change: preserving complete alternate-screen history reads.
Description check	✅ Passed	The description directly explains the history-read fixes, affected cases, implementation approach, and validation results.
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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Actionable comments posted: 1

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Inline comments:
In `@src/terminal/history_read.rs`:
- Around line 63-66: Update the history-anchor lookup around history_anchor to
compare normalized identities from row_identities(history) against
previous_text[anchor] rather than raw row_text values, preserving the existing
overlap limit and alignment flow. Add a regression test covering chained
scrolling where the retained overlap row’s scrollbar cell differs from the
current snapshot.

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Repository UI

Review profile: CHILL

Plan: Advanced

Run ID: c4645970-07da-4e6b-b2e5-b153786e4504

📥 Commits

Reviewing files that changed from the base of the PR and between 9ad65d9 and c9f2f57.

📒 Files selected for processing (1)

• src/terminal/history_read.rs

Included review availability: Your plan provides up to 10 included reviews per hour; 9 remain after this review.

[greptile-apps]

Confidence Score: 5/5

The PR appears safe to merge; the previously reported ambiguous-shift data-loss issue is fixed, and no new actionable failures were established.

Summary

• Normalizes recognized right-edge scrollbars only for row comparison while retaining original cells.
• Uses distinct rows to establish scroll alignment and rejects repeated-only or competing alignments.
• Refreshes overlapping rows to recover text obscured by pinned headers.
• Adds regressions for scrollbar movement, repeated rows, pinned headers, wrap metadata, and ambiguous anchors.

Diagram

%%{init: {'theme': 'neutral'}}%%
flowchart LR
  A[Previous viewport] --> C[Normalize row identities]
  B[Scrolled viewport] --> C
  C --> D{One uniquely anchored shift?}
  D -->|No| E[Return Unaligned]
  D -->|Yes| F[Locate retained anchor]
  F --> G[Refresh overlap and prepend older rows]
  G --> H[Preserved history]

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_{Reviews (2) · Last reviewed commit: "fix: preserve complete alternate-screen ..."}

[ogulcancelik]

@coderabbitai review
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@ogulcancelik: I will review pull request #3979.

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