feat(editor): adopt a MIDI's own tempo map on import (Use-vs-Keep)

[ChrisBeWithYou]

DAW roadmap 3.2 (SONG-EDITOR-DAW-WORKSPACE-DESIGN.md §F3). Newly unblocked: core #796 (convert_midi_tempo_map) merged, which is the gate this slice waited on.

The gap

Importing a .mid as keys or drums baked note times but threw away the file's tempo map and time signatures — so every MIDI import landed with an implied 4/4 and no bars, no matter what the file said.

What this does

The keys/drums MIDI import now reads the SMF's real grid and, when it carries one, offers a Use MIDI tempo map / Keep project timing choice. The default is honest and never silent:

• Use when the project has no bars yet (the MIDI's grid is strictly more information).
• Keep when a timeline already exists — an audio-aligned grid is never stomped.

Applying runs through the existing TempoGridCmd (one undoable step that re-locks the loop onto the new grid); imported notes stay accurate either way. It degrades cleanly — a gridless MIDI, a GP import, or an older host without the core function simply shows no prompt. On a drum import the timing choice is chained before the unmapped-notes triage so the two dialogs never stack.

Backend (routes.py)

• Module-level _sanitize_midi_tempo_map — pure gate + shape: returns {tempos, time_signatures, beats} only when the map has ≥1 numbered downbeat, else {}. Module-level so it's unit-testable without a real .mid or the core lib.
• Module-level _safe_midi_tempo_map — wraps the core call; returns {} on ImportError (older host predating #796) or any extraction error, so an import never 500s over timing. track_index is forwarded (SMF type-2 independent timelines).
• import-keys-midi / import-drums-midi compute the map in the same worker as the note conversion (both endpoints rmtree the temp dir immediately after) and return it as tempo_map.

Frontend (screen.js)

• @pure:midi-tempo-choice block: project-grid detection (≥2 downbeats), beat-row sanitizing, the Use-vs-Keep default policy, and a summary string (e.g. 12 bars · 4/4 · 120 BPM).
• _maybeOfferMidiTempoMap — the radio dialog + TempoGridCmd apply.
• TempoGridCmd is now song-scoped (like the drum commands), matching the EditHistory contract comment that already lists "tempo grid" as song-scope. Without it, applying an imported grid right after a drums import — active part still fretted-in-roll — would be silently blocked by the feat(editor): per-part view switcher — any fretted part opens in the piano roll (read-first) #119 read-only-roll lock. A latent bug this surfaces and fixes.

Tests

• tests/test_midi_tempo_import.py (8) — gate/shape purity, gridless drop, the two core-call failure floors (older core → {}, extraction raising → {}) via injection, track_index forwarding.
• tests/midi_tempo_import.test.js (10) — grid detection, sanitizing, default policy, summary, TempoGridCmd songScope + S.beats round-trip.
• Full JS suite green; editor pytest 221 passed / 2 skipped. Both features absent on main.

Scope

Single-track, riding the existing keys/drums pickers — the multitrack mapping-table picker + classifier is 3.3 (separate PR). Provenance (import_source) is 3.3 too. Fresh region: new module-level helpers among the _timeline_* family; the frontend block sits by the drum-import modals.

Summary by CodeRabbit

• New Features

  • MIDI imports can now optionally adopt the source file’s tempo/time-signature grid via a prompt to use MIDI timing or keep project timing.
  • The choice is applied as a single, undoable song-scoped change, and the tempo prompt is only shown when a usable grid is detected.
  • Drum MIDI import prioritizes this prompt to prevent overlapping dialogs.

• Bug Fixes

  • Improved safety and fallback when tempo-map data is missing, invalid, unsupported, or fails to extract.
  • Correctly applies audio offset to the adopted timing grid.

• Tests

  • Added/expanded frontend and Python coverage for tempo-mapped MIDI import gating, formatting, and offset shifting.

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Review details

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro Plus

Run ID: d72f3365-5eb8-4673-a5a3-598b9fbcc9b2

📥 Commits

Reviewing files that changed from the base of the PR and between f332394 and 0d12059.

📒 Files selected for processing (2)

• CHANGELOG.md
• screen.js

📝 Walkthrough

Walkthrough

Adds MIDI tempo-map import support in the backend and frontend, including optional adoption of the imported timing grid, song-scoped tempo grid application, and coverage for extraction, selection, and rollback behavior.

Changes

MIDI Tempo Map Import

Layer / File(s)	Summary
Backend tempo-map extraction and API responses routes.py	Adds safe tempo-map extraction, sanitization, and offset shifting, then includes tempo_map in the keys and drums MIDI import responses.
Frontend tempo-map adoption prompt and TempoGridCmd scoping screen.js	Adds tempo-map helper functions, the modal that offers MIDI timing adoption, import-flow sequencing for that modal, and TempoGridCmd.songScope = true.
Tests and changelog for tempo map import tests/midi_tempo_import.test.js, tests/test_midi_tempo_import.py, CHANGELOG.md	Adds JS and Python coverage for tempo-map shaping, wrapper behavior, modal logic, command behavior, and documents the change in the changelog.

Estimated code review effort: 3 (Moderate) | ~25 minutes

Sequence Diagram(s)

sequenceDiagram
  participant routes.py
  participant screen.js
  participant User
  participant TempoGridCmd
  participant import endpoint
  routes.py-->>screen.js: tempo_map in import response
  screen.js->>User: offer "Use this MIDI's timing?"
  User-->>screen.js: Apply or Skip
  screen.js->>TempoGridCmd: exec on Apply
  TempoGridCmd-->>screen.js: update S.beats
  screen.js-->>import endpoint: continue import flow

Loading

Possibly related PRs

• got-feedBack/feedBack-plugin-editor#92: Modifies TempoGridCmd behavior in screen.js, which is the command now used to apply imported MIDI tempo maps.

🚥 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 change: importing and optionally adopting a MIDI file’s tempo map in the editor.
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-midi-tempo-import

---

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Actionable comments posted: 2

🤖 Prompt for all review comments with AI agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Inline comments:
In `@routes.py`:
- Around line 410-436: The MIDI tempo map returned by _safe_midi_tempo_map is
still in raw MIDI time, while the imported notes/hits are already shifted by
audio_offset, so the grid and chart can diverge on offset imports. Update the
import flow where the result is passed into TempoGridCmd to apply audio_offset
to the tempo-map timing data first, adjusting the beat rows and any matching
tempos/time_signatures timestamps consistently. Use _safe_midi_tempo_map,
audio_offset, and TempoGridCmd as the main points to locate the change.

In `@screen.js`:
- Around line 14360-14399: The default-choice logic in
_midiTempoDefaultChoicePure is too strict for imported MIDI because it uses
_hasProjectGridPure, which requires two downbeats and therefore hides valid
single-bar tempo maps. Update the MIDI-side availability check to treat a
sanitized tempo map with at least one valid downbeat as offerable, while keeping
the stricter _hasProjectGridPure check for the existing projectBeats. Preserve
the current keep-vs-midi decision structure in _midiTempoDefaultChoicePure, but
base “nothing to offer” on a looser imported-grid predicate.

🪄 Autofix (Beta)

Fix all unresolved CodeRabbit comments on this PR:

• Push a commit to this branch (recommended)
• Create a new PR with the fixes

---

ℹ️ Review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro Plus

Run ID: 7868b2a2-5d62-4565-b98c-aef9af035fc0

📥 Commits

Reviewing files that changed from the base of the PR and between 8eae01b and b3a2f57.

📒 Files selected for processing (5)

• CHANGELOG.md
• routes.py
• screen.js
• tests/midi_tempo_import.test.js
• tests/test_midi_tempo_import.py

[byrongamatos]

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[byrongamatos]

Both CodeRabbit findings were already addressed in this PR before the review landed:

• Shift MIDI tempo map by audio_offset — fixed in _shift_midi_tempo_map() (routes.py); the adopted grid now rides the same offset the note converters apply, and both import endpoints pass audio_offset.
• Preserve single-bar MIDI grids — fixed via _midiOffersGridPure() (screen.js); the frontend now offers any map with ≥1 numbered downbeat, matching the backend gate.

Also folded time-signature denominators forward across downbeats so a persistent non-4 denominator survives adopt/save. Regression tests added on both sides (JS + pytest). Resolving the threads.