Drag previews clipped by Popup that created them

[rokiyo]

Godot version

4.0.beta2

System information

Windows 10

Issue description

Calling set_drag_preview() from a Control nested within an embedded PopupPanel will cause the preview to be clipped by that PopupPanel.

Seems possible that #64949 might fix this by allowing us to set the parent to the main viewport, but I don't understand the issue well enough to be sure about that.

Steps to reproduce

Some Controls nested within a PopupPanel:

The GridContainer in the image above has a script that instantiates each inventory slot as a scene, which looks like this:

The InventorySlotControl's script contains the following code:

func _get_drag_data(_position: Vector2):
	if item_stack is ItemStack:
		drag_data = { "item_stack": item_stack, "previous_slot": self }
		set_drag_preview(stack_container.duplicate())
		stack_taken.emit(get_index())
		return drag_data

func _can_drop_data(_position: Vector2, data) -> bool:
	return data is Dictionary and data.has("item_stack")

func _drop_data(_position: Vector2, data) -> void:
	stack_received.emit(get_index(), data)
	data.previous_slot.drag_data = null

func _unhandled_input(event: InputEvent) -> void:
	if event.is_action_released("ui_left_mouse") and drag_data is Dictionary:
		accept_event()
		stack_received.emit(get_index(), drag_data)
		drag_data = null

Minimal reproduction project

No response

[rokiyo]

I also saw this mentioned by another user on the Godot Discord the other day:

They seemed to eventually uncover this as a potential root cause:

godot/scene/main/viewport.cpp

Line 2131 in e5aa5eb

p_base->get_root_parent_control()->add_child(p_control); // Add as child of viewport.