Conflict between sequencer transform track and Parent Constraint from the Animation Mode breaks temporary pivot translation on any actor

Note: This affects all versions from at least UE 5.3, including mainline latest.

The "Animation" Editor Mode allows creating constraints for level actors. One of these, the "Parent" constraint, constrains the currently selected actor so that its relative transform to a given parent remains unchanged (much like attaching actors or components directly). If an actor subject to such a constraint also has a Transform Track on an open Sequencer (even if that Transform Track has no keys), this breaks using Alt+MMB+Drag to temporarily translate the pivot of any actor in the scene (even completely unrelated actors). While attempting to drag the temporary pivot, the translate widget can be seen jumping back to the original pivot location on every frame.

A preliminary investigation showed a potential cause of this problem. We have the following situation:

(1) Once a "parent" constraint is active, every editor tick executes the following call chain:

• FConstraintTickFunction::ExecuteTick()
• FConstraintTickFunction::EvaluateFunctions()
• UTickableTransformConstraint::SetChildGlobalTransform()
• UTransformableComponentHandle::SetGlobalTransform()
• FConstraintsManagerController::Get(World).OnSceneComponentConstrained().Broadcast()

(2) Once an F3DTransformTrackEditor is active, it does:

• UMovieScene3DTransformSection::ConstraintChannelAdded().AddRaw(this, &F3DTransformTrackEditor::HandleOnConstraintAdded);
  • Inside the delegate:
  • FConstraintsManagerController::Get(World).OnSceneComponentConstrained().AddLambda()
    • Inside the lambda:
    • GUnrealEd->UpdatePivotLocationForSelection()
      • This resets the pivot every frame

Notes:

• FConstraintsManagerController::OnSceneComponentConstrained() is broadcast every frame, even when there is no change resulting from applying the constraint
• The lambda registered from F3DTransformTrackEditor resets the pivot every frame even if an unrelated actor is selected, and even while not manipulating the viewport widget in any way.
• Deleting the (empty) transform track from the constrained actor makes the conflict stop

1. Create and open a new Level from the Basic template
2. Create and open a new Level Sequence
3. Create two spawnables
3.1. (e.g.) Level Editor toolbar – Place Actors – Shapes – Cube
3.2. (e.g.) Level Editor toolbar – Place Actors – Shapes – Sphere
4. In the Level Editor toolbar, change the Editor Mode from "Selection" to "Animation"
5. Select one of the spawnables
6. In the bottom of the "Animation" tab, click the Plus icon to create a new constraint
6.1. Select "Parent"
6.2. On the viewport, click on the other spawnable to set it as the constraint reference
7. Use Alt + MMB + Drag to temporarily translate the pivot of any actor (even an unrelated one, like the floor)
7.1. The pivot can be seen being reset every frame
8. The problem stops if any one of the following is done:
8A. Sequencer tab is closed
8B. Constraint is deleted
8C. Transform track on the constrained actor is removed from the sequence