refactor(editor): move Sync-Tempo to src/sync-tempo.js (R2, step 33)

[byrongamatos]

What

Step 33 of the R2 module-extraction refactor: move the Sync-Tempo feature out of src/main.js into a new native ES module src/sync-tempo.js.

Detect the audio's BPM (onset autocorrelation), compare against the tab's grid BPM, and scale notes/beats/sections to match — reprojecting onto the warped grid when a sync point is locked.

How the seam is drawn

• 4 window.editor* handlers (editorSyncTempo / editorSyncUpdateFactor / editorHideSyncDialog / editorApplySync) become named exports, re-attached by main.js.
• getTabBPM exported back — main.js calls it at 2 sites.
• The start-landing helpers (editorShowStartLanding / _editorMaybeShowStartLanding) that happened to share the banner are a separate feature and stayed in main.js — this was a function-level cut, not a banner range.
• draw routes through host; no new host hook.
• Removed the now-dead beatOf/timeOf main.js import (only the moved reproject used them).
• tests/beat_lock.test.mjs retargeted to slice editorApplySync's section-scale loop from src/sync-tempo.js (byte-identical).

main.js drops ~218 lines. 34 modules.

Verification

• 90/90 JS suites pass; ESLint gate clean (0 errors); strict no-undef clean on sync-tempo.js.
• Codex preflight: 0 correctness issues.
• New headless harness (verify_sync_tempo.py) drives the real detect→apply flow: editorSyncTempo detects the audio BPM by autocorrelation over the decoded buffer (90 tab / 120 audio on the diagnostic song) and populates the dialog; editorApplySync scales by the 1.3333× factor and reports it (proving host.draw resolved); the dialog closes; zero page errors. Negative-checked by breaking the host.draw call.

🤖 Generated with Claude Code

Summary by CodeRabbit

• New Features

  • Added a Sync Tempo workflow for matching editor beats and musical timing to an audio track.
  • Detects audio BPM and calculates tab BPM, with support for manual BPM and offset adjustments.
  • Preserves locked sync points while updating beats, notes, sustains, and section timing.

• Bug Fixes

  • Improved timing alignment when applying tempo synchronization with locked points.

• Tests

  • Updated beat-lock tests to validate the tempo synchronization workflow.

[coderabbitai]

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⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro Plus

Run ID: 493e6656-0263-42a3-9498-3afa6c2cef4a

📥 Commits

Reviewing files that changed from the base of the PR and between c69c425 and db39123.

📒 Files selected for processing (3)

• src/main.js
• src/sync-tempo.js
• tests/beat_lock.test.mjs

⏰ Context from checks skipped due to timeout. (1)

• GitHub Check: CodeRabbit / Review

🧰 Additional context used

🪛 ast-grep (0.44.1)

src/sync-tempo.js

[error] 135-135: React's useState should not be directly called
Context: setStatus('Need audio and beats loaded for sync')
Note: [CWE-710] Improper Adherence to Coding Standards. Security best practice.

(usestate-direct-usage)

---

[error] 139-139: React's useState should not be directly called
Context: setStatus('Detecting audio BPM...')
Note: [CWE-710] Improper Adherence to Coding Standards. Security best practice.

(usestate-direct-usage)

---

[error] 155-155: React's useState should not be directly called
Context: setStatus('Ready')
Note: [CWE-710] Improper Adherence to Coding Standards. Security best practice.

(usestate-direct-usage)

---

[error] 224-224: React's useState should not be directly called
Context: setStatus(Tempo synced: scaled ${factor.toFixed(4)}x + (offset ? , offset ${offset}s : ''))
Note: [CWE-710] Improper Adherence to Coding Standards. Security best practice.

(usestate-direct-usage)

📝 Walkthrough

Walkthrough

Sync Tempo functionality moves from main.js into sync-tempo.js, which now handles BPM detection, dialog controls, locked-grid rescaling, note and section reprojection, and exported editor actions. Beat-lock tests now extract the section-scaling logic from the new module.

Changes

Sync Tempo modularization

Layer / File(s)	Summary
Extract and wire Sync Tempo src/main.js, src/sync-tempo.js	Tempo-sync logic is moved into sync-tempo.js; main.js imports the exports and re-attaches the expected window globals.
Detect tempo and manage sync dialog src/sync-tempo.js	Audio BPM detection, tab BPM calculation, factor updates, and dialog visibility are implemented in the new module.
Apply locked grid synchronization src/sync-tempo.js, tests/beat_lock.test.mjs	Beat, note, and section timing is rescaled with lock-aware respacing, and the beat-lock test harness extracts the section logic from sync-tempo.js.

Estimated code review effort: 4 (Complex) | ~45 minutes

Sequence Diagram(s)

sequenceDiagram
  participant EditorUI
  participant sync-tempo.js
  participant S.audioBuffer
  participant S.beats
  participant host.draw
  EditorUI->>sync-tempo.js: editorSyncTempo()
  sync-tempo.js->>S.audioBuffer: detectAudioBPM()
  sync-tempo.js->>S.beats: getTabBPM()
  sync-tempo.js->>EditorUI: show BPM and factor dialog
  EditorUI->>sync-tempo.js: editorApplySync()
  sync-tempo.js->>S.beats: rescale beats, notes, and sections
  sync-tempo.js->>host.draw: redraw editor

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Possibly related PRs

• got-feedBack/feedBack-plugin-editor#138: Implements related beat-lock re-fit mechanics used by the extracted synchronization flow.
• got-feedBack/feedBack-plugin-editor#161: Extracts beatOf and timeOf, which the new tempo module uses.
• got-feedBack/feedBack-plugin-editor#170: Moves related tempo-map and beat-lock respacing logic into a separate module.

🚥 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 describes the main refactor of moving Sync Tempo into a new module.
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 refactor/r2-step33-sync-tempo

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