Event delivery does not work well with subScenes.

Events are delivered inside subScenes, but they don't bubble through subScene parents, not even the subScene itself. So it is impossible to implement event filters and default handlers for subScene and its parents.

Then there is a problem with event local coordinates. They use the localToScene and sceneToLocal methods to compute the coordinates from the intersection point. This won't work outside of the intersected subScene because the methods compute only to its root.
Note that during mouse dragging, we may need to deliver the event to a target node living in a different subScene than the picked one, and the coordinates must be expressed in the space of the event target. In complex 3D cases this is hard to even define given the non-invertible subScene flattening transform in the way.