Bug: Undo undoes multiple changes despite pauses between them

This might just be due to an automatic edit run merge or something, but:

- Open a completed job in the transcribee editor
- Edit some word
- Wait for a few seconds
- Then jump to another word by clicking a word or selecting a substring and _quickly_ start typing
- Hit undo

Actual:

- Your recent typing _and_ your previous edit are undone

Expected:

- Undo should undo only the last edit. Repositioning the cursor or pausing editing for a certain amount of time should result in a separate transformation event to be undone.

Reproduce the issue in the transcribee editor using a completed job, a paused edit, and a quick edit after repositioning the cursor. Trace the undo handling and transformation-event grouping used by the editor. Done means undo removes only the most recent edit, while pauses or cursor repositioning create separate undoable events.

Written by the indexing model from the issue text.