Fix drag notifications across windows.

[reduz]

Fixes #44390.

[Sauermann]

This is an alternative approach to #67531.

From what I can tell, this PR correctly solves a single problem (highlighting valid Drop locations of child Windows and SubViewports) of the many issues related to Drag and Drop with Windows and SubViewports, while the other PR intends to solve all of them.

Since the other PR no longer makes use of _propagate_viewport_notification it could be considered as superseding this PR.

Here is a MRP for stress-testing Drag and Drop with Windows and SubViewports BugDragNDrop.zip

[reduz]

@Sauermann This fixes sub-viewports too (although i realized a problem in this PR that needs to be further fixed) done. The proper handling of the drag preview needs to be done by moving it to a transparent window with passthrough (like tooltips), but that should be implemented in a separate PR.

[kleonc]

Suggested change

	base = this;
	base = vp;

[kleonc]

Not an issue but do we try to be consistent with the checks style?:thinking: I mean I get there are different preferences but you're doing it both ways in this PR (if (vp) and if (vp != nullptr)) so that's kinda awkward. 😄

[Sauermann]

I wrote #71509 as a far simpler way to solve this. I believe It fixes drags/drops accross viewports more elegantly.

After investigating this topic more deeply, one problem area, that I was able to identify is, that the Viewport, that the mouse is over, is identified in the following code section:

godot/scene/main/viewport.cpp

Lines 1830 to 1845 in 23e10d1

	// Use DisplayServer logic.
	Vector2i screen_mouse_pos = DisplayServer::get_singleton()->mouse_get_position();
	DisplayServer::WindowID window_id = DisplayServer::get_singleton()->get_window_at_screen_position(screen_mouse_pos);
	if (window_id != DisplayServer::INVALID_WINDOW_ID) {
	ObjectID object_under = DisplayServer::get_singleton()->window_get_attached_instance_id(window_id);
	if (object_under != ObjectID()) { // Fetch window.
	Window *w = Object::cast_to<Window>(ObjectDB::get_instance(object_under));
	if (w) {
	viewport_under = w;
	viewport_pos = screen_mouse_pos - w->get_position();
	}
	}
	}

And this code section doesn't account for SubViewports and embedded Windows.
My conclusion is, that in order to fix Drag and Drop for Viewports, it is necessary make this code-section aware of SubViewports and embedded Windows. This conclusion aligns with my tests of this PR with the previously linked MRP, which show that Drag and Drop with Viewports is not yet solved completely.

[YuriSizov]

Needs some consensus and conclusion. We can pick it for 4.0.x though, so bumping for now.

[akien-mga]

Superseded by #67531.