perf(ui): avoid full pane mode reads for scrollbars

[akbash-bot]

Summary

• query only authoritative alternate-screen state when computing scrollbar gutters
• avoid collecting unrelated terminal input modes for every background pane during full renders
• preserve primary/alternate-screen gutter and background resize behavior

Checks

• cargo test alternate_screen_reclaims_scrollbar_gutter_and_restores_it_on_exit
• live 120x40 PTY profile: one workspace 6.17–6.45 ms; 15 workspaces 6.61–7.43 ms steady full_render.render_virtual averages
• cargo fmt --check and cargo clippy --all-targets --locked -- -D warnings
• local just check nextest run reached 963 passing tests, then hit the unrelated live_server_holds_one_pty_master_fd_per_pane helper assumption that the binary lives below checkout target/debug; Kennel uses an external isolated CARGO_TARGET_DIR

refs #2550

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro Plus

Run ID: 40886739-ed2c-4f8f-8a00-1624a358171d

📥 Commits

Reviewing files that changed from the base of the PR and between d277d2f and 6a5ad18.

📒 Files selected for processing (4)

• src/pane.rs
• src/pane/terminal.rs
• src/terminal/runtime.rs
• src/ui/panes.rs

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📝 Walkthrough

Walkthrough

The change adds alternate-screen state accessors across terminal runtime layers. terminal_inner_rect now uses the runtime state instead of input state when reserving the scrollbar column.

Changes

Alternate screen state

Layer / File(s)	Summary
Terminal state accessor src/pane/terminal.rs	PaneTerminal and GhosttyPaneTerminal expose alternate_screen_active(). The lower-level accessor returns false when state access fails.
Runtime and layout integration src/pane.rs, src/terminal/runtime.rs, src/ui/panes.rs	Runtime layers delegate alternate-screen queries. terminal_inner_rect uses the runtime query for scrollbar-gutter behavior.

Estimated code review effort: 2 (Simple) | ~10 minutes

Possibly related PRs

• herdrdev/herdr#2086: Uses alternate-screen detection for scrollback-preservation logic.
• herdrdev/herdr#2538: Also changes scrollbar-gutter behavior in src/ui/panes.rs based on alternate-screen state.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 7.14% which is insufficient. The required threshold is 80.00%.	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 summarizes the main performance change: avoiding full pane mode reads during scrollbar computation.
Description check	✅ Passed	The description directly explains the scrollbar optimization, preserved behavior, performance checks, and test 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.

✨ Finishing Touches 💡 1

📝 Generate docstrings 💡

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch akbash/2550-fast-scrollbar-state-read

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