[Bug]: MP4 export cannot be cancelled — Cancel is disabled while rendering

[EtienneLescot]

What happens

During an MP4 export the dialog's Cancel button is greyed out and does nothing; three real clicks between 13 % and 62 % progress were ignored and the export ran to completion. GIF export's Cancel works (returns to the options, leaves no partial file).

Expected

MP4 export can be cancelled mid-render like GIF export (checklist "Export progress and speed-region audio", line 578; "Export", line 559).

Steps

1. Open a project, Export → MP4 (720p/24 here).
2. Save, then click Cancel while "Exporting your video" advances.

Environment

v2.0.0 (DMG), macOS 27.0, Mac mini M1 (native compositor export). Main-process thread count returned to baseline after the export finished.

[github-actions]

Fixed by #1044 and merged into main.

This change is assigned to the Next Release release milestone and is not necessarily available in the latest downloadable desktop release yet. It is currently marked as status: pending release until a packaged release containing it is published.