Allow drag_preview to be handled by a different viewport than its parent

doc/classes/Control.xml

@@ -336,9 +336,11 @@
 			<return type="void" />
 			<param index="0" name="data" type="Variant" />
 			<param index="1" name="preview" type="Control" />
+			<param index="2" name="parent" type="Control" default="null" />
 			<description>
 				Forces drag and bypasses [method _get_drag_data] and [method set_drag_preview] by passing [param data] and [param preview]. Drag will start even if the mouse is neither over nor pressed on this control.
 				The methods [method _can_drop_data] and [method _drop_data] must be implemented on controls that want to receive drop data.
+				The [param parent] parameter defines which control will be used to determine preview's parent, or defaults to [code]this[/code] if left null.
 			</description>
 		</method>
 		<method name="get_anchor" qualifiers="const">
@@ -844,6 +846,7 @@
 		<method name="set_drag_preview">
 			<return type="void" />
 			<param index="0" name="control" type="Control" />
+			<param index="1" name="parent" type="Control" default="null" />
 			<description>
 				Shows the given control at the mouse pointer. A good time to call this method is in [method _get_drag_data]. The control must not be in the scene tree. You should not free the control, and you should not keep a reference to the control beyond the duration of the drag. It will be deleted automatically after the drag has ended.
 				[codeblocks]
@@ -873,6 +876,7 @@
 				}
 				[/csharp]
 				[/codeblocks]
+				The [param parent] parameter defines which control will be used to determine preview's parent, or defaults to [code]this[/code] if left null.
 			</description>
 		</method>
 		<method name="set_end">

editor/plugins/visual_shader_editor_plugin.cpp

@@ -4474,7 +4474,7 @@ Variant VisualShaderEditor::get_drag_data_fw(const Point2 &p_point, Control *p_f
 
 		Label *label = memnew(Label);
 		label->set_text(it->get_text(0));
-		set_drag_preview(label);
+		members->set_drag_preview(label, this);
 		return d;
 	}
 	return Variant();

scene/gui/control.cpp

@@ -1810,17 +1810,26 @@ void Control::drop_data(const Point2 &p_point, const Variant &p_data) {
 	GDVIRTUAL_CALL(_drop_data, p_point, p_data);
 }
 
-void Control::force_drag(const Variant &p_data, Control *p_control) {
+void Control::force_drag(const Variant &p_data, Control *p_control, Control *p_parent) {
 	ERR_FAIL_COND(!is_inside_tree());
 	ERR_FAIL_COND(p_data.get_type() == Variant::NIL);
 
-	get_viewport()->_gui_force_drag(this, p_data, p_control);
+	if (p_parent == nullptr) {
+		p_parent = this;
+	}
+
+	get_viewport()->_gui_force_drag(p_parent, p_data, p_control);
 }
 
-void Control::set_drag_preview(Control *p_control) {
+void Control::set_drag_preview(Control *p_control, Control *p_parent) {
 	ERR_FAIL_COND(!is_inside_tree());
 	ERR_FAIL_COND(!get_viewport()->gui_is_dragging());
-	get_viewport()->_gui_set_drag_preview(this, p_control);
+
+	if (p_parent == nullptr) {
+		p_parent = this;
+	}
+
+	get_viewport()->_gui_set_drag_preview(p_parent, p_control);
 }
 
 bool Control::is_drag_successful() const {
@@ -3423,7 +3432,7 @@ void Control::_bind_methods() {
 	ClassDB::bind_method(D_METHOD("set_focus_previous", "previous"), &Control::set_focus_previous);
 	ClassDB::bind_method(D_METHOD("get_focus_previous"), &Control::get_focus_previous);
 
-	ClassDB::bind_method(D_METHOD("force_drag", "data", "preview"), &Control::force_drag);
+	ClassDB::bind_method(D_METHOD("force_drag", "data", "preview", "parent"), &Control::force_drag, DEFVAL((Control *)nullptr));
 
 	ClassDB::bind_method(D_METHOD("set_mouse_filter", "filter"), &Control::set_mouse_filter);
 	ClassDB::bind_method(D_METHOD("get_mouse_filter"), &Control::get_mouse_filter);
@@ -3437,7 +3446,7 @@ void Control::_bind_methods() {
 	ClassDB::bind_method(D_METHOD("grab_click_focus"), &Control::grab_click_focus);
 
 	ClassDB::bind_method(D_METHOD("set_drag_forwarding", "target"), &Control::set_drag_forwarding);
-	ClassDB::bind_method(D_METHOD("set_drag_preview", "control"), &Control::set_drag_preview);
+	ClassDB::bind_method(D_METHOD("set_drag_preview", "control", "parent"), &Control::set_drag_preview, DEFVAL((Control *)nullptr));
 	ClassDB::bind_method(D_METHOD("is_drag_successful"), &Control::is_drag_successful);
 
 	ClassDB::bind_method(D_METHOD("warp_mouse", "position"), &Control::warp_mouse);

scene/gui/control.h

@@ -501,8 +501,8 @@ class Control : public CanvasItem {
 	virtual Variant get_drag_data(const Point2 &p_point);
 	virtual bool can_drop_data(const Point2 &p_point, const Variant &p_data) const;
 	virtual void drop_data(const Point2 &p_point, const Variant &p_data);
-	void set_drag_preview(Control *p_control);
-	void force_drag(const Variant &p_data, Control *p_control);
+	void set_drag_preview(Control *p_control, Control *p_parent = nullptr);
+	void force_drag(const Variant &p_data, Control *p_control, Control *p_parent = nullptr);
 	bool is_drag_successful() const;
 
 	// Focus.

scene/main/viewport.cpp

@@ -1779,7 +1779,14 @@ void Viewport::_gui_input_event(Ref<InputEvent> p_event) {
 
 			Control *drag_preview = _gui_get_drag_preview();
 			if (drag_preview) {
-				drag_preview->set_position(mpos);
+				// The viewport which started the drag takes care of the preview position,
+				// even if it's owned by another viewport. So, we need to convert the mouse
+				// position according to the target viewport.
+
+				Point2i this_position = get_base_window()->get_position();
+				Point2i owner_position = drag_preview->get_viewport()->get_base_window()->get_position();
+
+				drag_preview->set_position(mpos + this_position - owner_position);
 			}
 
 			gui.drag_mouse_over = over;