Fixes T10912. When you drag tasks within a milestone, we currently apply an overbroad, API-focused rule and add the parent board's project. This logic was added fairly recently, as part of T6027, to improve the behavior of API-originated moves.
Later on, this causes the task to toggle in and out of the parent project on every alternate drag.
This logic is also partially duplicated in the MoveController.
- Add test coverage for this interaction.
- Fix the logic so it accounts for subproject / milestone columns correctly.
- Put all of the logic into the TransactionEditor, so the API gets the exact same rules.