fix(editor): flatten a variable tempo map to a constant BPM

[ChrisBeWithYou]

What & why

A tester reported that a variable-tempo import (a GP8 file that came in with a
wobbling per-measure tempo map) had "no way to remove all the BPM sync points
without deleting measures." The per-measure BPM editor only fixes one measure
at a time, and typing a BPM on such a song just refused: "Use Tempo Map to edit
songs with multiple tempo events."

This turns that dead end into an escape hatch: typing a BPM on a variable
tempo map now offers to flatten the whole map to that one constant BPM.

How

• _tempoFlattenToBpmPure (@pure:tempo-map-bpm) rebuilds the beat grid as
  a uniform 60/bpm grid — each beat re-timed to t0 + i·(60/bpm), keeping
  every beat's measure / time-signature metadata and the first beat's start
  time. Spacing by 60/bpm matches how the editor already defines a measure's
  BPM (beatCount·60/span, see _tempoMeasureBpmsPure). Spacing is exact
  (unrounded) on purpose — _r3's ±0.5 ms would drift the per-measure BPM
  past the 0.01 variable-tempo detector and the grid would still read as
  variable.
• window.editorSetBPM: on a variable map, confirm() then flatten via one
  undoable TempoGridCmd. Notes are not touched — they keep their times so
  they stay aligned to any audio; undo restores the original tempo map.

Tests

tests/tempo_flatten.test.js (5) — the sample grid really is a variable map,
flatten yields a uniform 60/bpm grid with metadata + origin preserved, the
result round-trips to a constant BPM through the reader, a non-zero origin is
kept, and the adversarial guards (too-few beats / bad bpm / null) return null
without throwing. All fail on main. Full JS suite green (72/72).

Summary by CodeRabbit

• New Features

  • When entering a BPM for songs with variable tempo maps, the editor now prompts to flatten the entire tempo map to a single constant BPM.
  • Flattening rebuilds the beat grid using uniform BPM spacing while preserving measure/time-signature info and the original start offset, keeping alignment with audio.

• Bug Fixes

  • Fixes an issue where BPM sync points couldn’t all be removed without deleting measures.
  • The flattening change is now captured as a single undoable action and reports completion after success.

[coderabbitai]

Caution

Review failed

The pull request is closed.

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro Plus

Run ID: eeb58574-56bb-49f2-9a71-7952a8dbe088

📥 Commits

Reviewing files that changed from the base of the PR and between be13f61 and c15e967.

📒 Files selected for processing (3)

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

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📝 Walkthrough

Walkthrough

Adds BPM-driven flattening for variable tempo maps, using a new pure helper, an undoable TempoGridCmd flow, validation tests, and a changelog update.

Changes

Tempo Map Flattening

Layer / File(s)	Summary
Flatten confirmation flow and undo integration screen.js	BPM entry on variable-tempo songs now prompts to flatten the map to a constant BPM, applies the change through TempoGridCmd, updates the BPM display, redraws, or exits early on rejection or failure.
Pure flatten-to-BPM computation screen.js	Adds _tempoFlattenToBpmPure, which validates beats and BPM inputs, computes uniform spacing from 60/bpm, and returns a cloned beats array with retimed entries or null on invalid input.
Test harness and flatten validation tests tests/tempo_flatten.test.js	Adds a Node test runner that extracts the pure tempo helpers from screen.js, builds a variable-tempo fixture, and verifies flattening behavior, metadata preservation, origin anchoring, and invalid-input handling.
Changelog entry CHANGELOG.md	Adds a changelog entry describing the tempo flattening behavior and the referenced test file.

Estimated code review effort: 2 (Simple) | ~15 minutes

Sequence Diagram(s)

sequenceDiagram
  participant User
  participant screenjs as screen.js
  participant tempoFlatten as _tempoFlattenToBpmPure
  participant tempoGridCmd as TempoGridCmd

  User->>screenjs: Type BPM value
  screenjs->>screenjs: Detect variable tempo map
  screenjs->>User: Confirm flatten whole map
  User->>screenjs: Accept
  screenjs->>tempoFlatten: Flatten beats to constant BPM
  tempoFlatten-->>screenjs: New beats array or null
  alt flatten succeeded
    screenjs->>tempoGridCmd: Record tempo map change
    screenjs->>screenjs: Update BPM display and redraw
  else flatten failed or declined
    screenjs->>screenjs: Update BPM display and return
  end

Loading

Possibly related PRs

• got-feedBack/feedBack-plugin-editor#127: Also changes TempoGridCmd and the screen.js tempo-grid editing path around applying tempo map data.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 50.00% 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 clearly summarizes the main change: flattening variable tempo maps to a constant BPM in the editor.
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/editor-tempo-flatten

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