_can_drop_data not called in sub-viewports

[Splagoon]

Godot version

v4.0.beta3.official [01ae26d]

System information

Windows 10

Issue description

When dragging and dropping controls inside a SubViewport (inside a SubViewportContainer), dragging a control will call _get_drag_data as expected, but dragging over other controls never calls _can_drop_data. This was not the case in Godot 3.5, where dragging and dropping controls in a ViewportContainer worked as expected.

Steps to reproduce

Two sample projects are included:

• DragAndDropRepro3 was built using v3.5.1.stable.official [6fed1ffa3] and works as expected
• DragAndDropRepro4 was built using v4.0.beta3.official [01ae26d31] and calls _get_drag_data as expected but never calls _can_drop_data nor _drop_data

Minimal reproduction project

DragAndDropRepro.zip

[Sauermann]

Will be solved by #67531

[KoBeWi]

Duplicate of #28522