fix(scroll): keep one staff-system row of lookahead visible

[gionnibgud]

Summary

Restores the lookahead auto-scroll lost in the clean-relaunch snapshot (the fix existed as 6121009 on the legacy history but predates the relaunch, so it never carried over). Reported as "chart not scrolling" — actually: no lookahead, the next row stayed hidden until the last note of the current row.

• Page layout: when the next staff system's bottom is below the viewport, scroll immediately so the current row sits at ~25% from the top, revealing the next row.
• Backward-seek guard (top 15% inset) and last-row centre fallback preserved.
• Horizontal layout unchanged.

Study mode gets the same behaviour for free: _svStudySnapCursor drives the same _svUpdateMarker, and the lookahead is deliberately not gated on _svStudyMode — covered by test.

Test plan

• tests/scroll.test.js: 4 branch tests + study/playback scroll-equivalence; verified non-vacuous (pre-fix code fails the lookahead test).
• npm test: 122/122.
• Manual: play a multi-line page-layout chart; the next system is visible before the current row's last note.

🤖 Generated with Claude Code

Summary by CodeRabbit

• Bug Fixes
  • Improved playback auto-scrolling to keep the current staff row visible and reveal the next row earlier.
  • Added smoother handling when navigating backward through the score.
  • Prevented unnecessary vertical scrolling in horizontal layouts.
  • Aligned scrolling behavior between normal playback and study mode.
  • Improved handling of the final staff row by avoiding attempts to reveal a nonexistent next row.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro Plus

Run ID: 02aa1610-e702-44f0-9281-3978e8ce295d

📥 Commits

Reviewing files that changed from the base of the PR and between 3cf5b38 and 1785b1d.

📒 Files selected for processing (2)

• screen.js
• tests/scroll.test.js

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📝 Walkthrough

Walkthrough

_svUpdateMarker() now uses page-layout row lookahead for vertical auto-scrolling while preserving horizontal scrolling. New tests validate playback, study mode, edge cases, and the absence of study-mode gating.

Changes

Marker auto-scroll

Layer / File(s)	Summary
Page-layout lookahead scrolling screen.js	Vertical scrolling uses the next staff system row in page layout, with fallback handling for the last row; horizontal pad-based scrolling remains unchanged.
Playback and study-mode scroll validation tests/scroll.test.js	Adds isolated harness tests for lookahead scrolling, last-row centering, backward seeks, horizontal layouts, study-mode equivalence, and source-level gating checks.

Estimated code review effort: 3 (Moderate) | ~20 minutes

Sequence Diagram(s)

sequenceDiagram
  participant Playback
  participant _svUpdateMarker
  participant _svApi
  participant _svContainer
  Playback->>_svUpdateMarker: Update marker for target row
  _svUpdateMarker->>_svApi: Find current and next staff bounds
  _svApi-->>_svUpdateMarker: Return row geometry
  _svUpdateMarker->>_svContainer: Scroll to calculated position

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Possibly related PRs

• got-feedBack/feedBack-plugin-staffview#15: Refactors _svUpdateMarker auto-scroll behavior and tests study-mode interactions.

Suggested reviewers: byrongamatos

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 66.67% 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
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title accurately captures the main change: preserving one staff-system row of lookahead during scroll behavior.
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/scroll-lookahead

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