Mixing Node2D and Control causes issues with set_as_toplevel

[Zoomulator]

Godot version:
3.2.3

OS/device including version:
Window 10, 19041.508

Issue description:
Input is not handled correctly in the provided reproduction project.

The project has a hierarchy Control -> Node2D -> Control.
Using set_as_toplevel(true) on the Node2D after adding it to the scene tree will cause the child Control to ignore input.

Removing and re-attaching the Node2D node will fix the behavior, letting the child Control receive input again.

If set_as_toplevel(true) is used before the Node2D is attached it works as expected.

Steps to reproduce:
See minimal reproduction.

• Try to click the "click me" control. It will flash red if it received the click, but it doesn't work.
• Click the top left button "Reattach"
• Click the "click me" control again, it should now flash red when clicked.

You can move the set_as_toplevel(true) line in Main.gd to before the add_child call and it will also work.

Minimal reproduction project:
toplevel-bug.zip

[gvaneyck]

Code reason this happens:

• Since the new node has a parent Control object and is not yet marked as top level, it does not get added as a gui root control (no call to get_viewport()->_gui_add_root_control(this)) in Control::_notification(NOTIFICATION_ENTER_CANVAS)
• CanvasItem::set_as_toplevel() calls CanvasItem::_exit_canvas() and CanvasItem::_enter_canvas() when the node is in the tree, but those will not do a Control::_notification, so the gui root control will not be attached (or detached)

[Sauermann]

Here is a MRP for Godot 4:
toplevel-bug-4.zip