feat(editor): Parts view — stacked overview of every part

[ChrisBeWithYou]

Summary

The editor shows one arrangement at a time through a bare <select> — there's no way to see the whole song. This adds the Parts view (Shift+A in both profiles, or the new "☰ Parts" toolbar button): every part as a named horizontal lane over one shared timeline, with one playhead sweeping all lanes during playback — the first slice of the workspace design's arrange window.

• One lane per part: every arrangement plus the drum tab (drums are a song-level sidecar, not an arrangement — they get their own lane). Lane headers show name, inferred instrument tag (Guitar/Bass/Keys/Drums), and event count; the armed part is highlighted.
• Compact silhouettes per kind: guitar/bass render string-ribbon rows (low strings at the bottom, matching the focus editor's orientation); keys render a pitch-mapped mini roll auto-fit to the part's range; drums collapse to the 3-row cymbals/drums/kick bands with piece colors. Non-active arrangements aren't chord-flattened, so silhouettes walk chords[].notes explicitly.
• Navigational by design (the overview navigates; focus editors edit): click a lane to arm it, double-click to open its focus editor — arrangements open the note editor, the drums lane enters the drum grid. Waveform clicks seek. Downbeat grid lines run through every lane.
• Mutually exclusive with drum-edit and Tempo Map modes (same pattern they use with each other); the toggle joins the command registry and the toolbar button follows the memoized-refresh pattern of the drum/tempo buttons.

Rename/reorder headers, mute/solo per lane, and coverage fills are the next slices on this view (2.2b in the design doc); they need the guide-engine per-part gains (#90) and the rename-safety work (type stamping) respectively.

Verification

• node --check screen.js clean
• New tests/parts_view.test.js — 5 cases via the @pure:parts-view block (part-list assembly incl. chord-instance counting and the drums lane, empty-drum-tab exclusion, lane-height clamps, 3-band drum mapping, lane hit-testing vs the waveform band)
• All 26 JS test files pass

🤖 Generated with Claude Code

https://claude.ai/code/session_01JEoFeTPSnz4NpwwCG52hnu

Summary by CodeRabbit

• New Features

  • Added a new Parts view showing arrangements as stacked lanes along the timeline, with playhead sweeping across all parts.
  • You can now single-click to arm a part and double-click to open its focused editor.
  • Added a Parts toggle button and keyboard shortcut for quickly switching views.
  • Parts view now stays separate from drum edit and tempo map modes.

• Tests

  • Added coverage for Parts view lane building, layout, and click/selection behavior.

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Review details

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro Plus

Run ID: 4a9413ec-ebc5-4dbc-a741-65f9e168472a

📥 Commits

Reviewing files that changed from the base of the PR and between c2e20fe and eaf53ca.

📒 Files selected for processing (3)

• CHANGELOG.md
• screen.js
• tests/parts_view.test.js

📝 Walkthrough

Walkthrough

Adds a new "Parts view" editor mode showing all parts as stacked lanes with playback playhead, click-to-arm and double-click-to-focus-edit interactions, a toolbar toggle button, Shift+A keyboard shortcut, mutual exclusivity with drum-edit/Tempo Map modes, plus tests and changelog entry.

Changes

Parts View Feature

Layer / File(s)	Summary
State flag and core implementation screen.js	Adds S.partsViewMode state, implements _partsViewDraw, _partsViewOnMouseDown, _partsViewOnDblClick, _editorTogglePartsView (exported as window.editorTogglePartsView), toolbar button management, and short-circuits draw() into the parts-view render path.
Mouse input routing screen.js	Routes onMouseDown and onDblClick into parts-view handlers when the mode is active, bypassing normal note/tone/anchor handlers.
Keyboard shortcut and command dispatch screen.js	Adds a togglePartsView shortcut command bound to Shift+A in both shortcut profiles and wires the EOF command switch to invoke _editorTogglePartsView().
Mode mutual exclusivity screen.js	Clears S.partsViewMode when entering drum edit or tempo-map mode and refreshes the relevant toggle buttons.
Tests and changelog tests/parts_view.test.js, CHANGELOG.md	Adds a Node test script exercising pure Parts view helper functions (list building, lane layout, drum band mapping, hit-testing) and documents the feature in the changelog.

Estimated code review effort: 3 (Moderate) | ~30 minutes

Sequence Diagram(s)

sequenceDiagram
  participant User
  participant Screen as screen.js draw loop
  participant PartsView as Parts View handlers
  participant Editor as Focus editor (arrangement/drum)

  User->>Screen: triggers draw()
  Screen->>PartsView: _partsViewDraw(w, h) when S.partsViewMode
  User->>Screen: mousedown on lane
  Screen->>PartsView: _partsViewOnMouseDown(e, x, y)
  PartsView-->>Screen: arms selected part
  User->>Screen: double-click on lane
  Screen->>PartsView: _partsViewOnDblClick(e)
  PartsView->>Editor: opens arrangement editor or enters drum edit mode

Loading

Possibly related PRs

• got-feedBack/feedBack-plugin-editor#73: Both PRs modify screen.js's Tempo Map mode toggle and button refresh logic, directly overlapping with this PR's mode-exclusivity changes that clear Parts view state on tempo-map entry.

🚥 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 clearly and concisely describes the main change: adding a stacked Parts view editor overview.
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 unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch feat/editor-parts-view

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