Enable Drag and Drop for SubViewports and Windows

Make Drag and Drop an application-wide operation.
This allows do drop on Controls in other Viewports/Windows.

In order to achieve this, `Viewport::_update_mouse_over` is adjusted to
remember the Control, that the mouse is over (possibly within nested
viewports). This Control is used as a basis for the Drop-operation, which
replaces the previous algorithm, which was only aware of the topmost
Viewport.

Also now all nodes in the SceneTree are notified about the Drag and Drop
operation, with the exception of SubViewports that are not children of
SubViewportContainers.

1 files changed, 131 insertions, 13 deletions

diff --git a/tests/scene/test_viewport.h b/tests/scene/test_viewport.h
index 1341cc0332..9d02c41719 100644
--- a/tests/scene/test_viewport.h
+++ b/tests/scene/test_viewport.h
@@ -119,8 +119,23 @@ public:
 class DragTarget : public NotificationControlViewport {
 	GDCLASS(DragTarget, NotificationControlViewport);
 
+protected:
+	void _notification(int p_what) {
+		switch (p_what) {
+			case NOTIFICATION_DRAG_BEGIN: {
+				during_drag = true;
+			} break;
+
+			case NOTIFICATION_DRAG_END: {
+				during_drag = false;
+			} break;
+		}
+	}
+
 public:
 	Variant drag_data;
+	bool valid_drop = false;
+	bool during_drag = false;
 	virtual bool can_drop_data(const Point2 &p_point, const Variant &p_data) const override {
 		StringName string_data = p_data;
 		// Verify drag data is compatible.
@@ -136,6 +151,7 @@ public:
 
 	virtual void drop_data(const Point2 &p_point, const Variant &p_data) override {
 		drag_data = p_data;
+		valid_drop = true;
 	}
 };
 
@@ -1107,12 +1123,10 @@ TEST_CASE("[SceneTree][Viewport] Controls and InputEvent handling") {
 	SUBCASE("[Viewport][GuiInputEvent] Drag and Drop") {
 		// FIXME: Drag-Preview will likely change. Tests for this part would have to be rewritten anyway.
 		// See https://github.com/godotengine/godot/pull/67531#issuecomment-1385353430 for details.
-		// FIXME: Testing Drag and Drop with non-embedded windows would require DisplayServerMock additions
-		// FIXME: Drag and Drop currently doesn't work with embedded Windows and SubViewports - not testing.
-		// See https://github.com/godotengine/godot/issues/28522 for example.
+		// Note: Testing Drag and Drop with non-embedded windows would require DisplayServerMock additions.
 		int min_grab_movement = 11;
-		SUBCASE("[Viewport][GuiInputEvent] Drag from one Control to another in the same viewport.") {
-			SUBCASE("[Viewport][GuiInputEvent] Perform successful Drag and Drop on a different Control.") {
+		SUBCASE("[Viewport][GuiInputEvent][DnD] Drag from one Control to another in the same viewport.") {
+			SUBCASE("[Viewport][GuiInputEvent][DnD] Perform successful Drag and Drop on a different Control.") {
 				SEND_GUI_MOUSE_BUTTON_EVENT(on_a, MouseButton::LEFT, MouseButtonMask::LEFT, Key::NONE);
 				CHECK_FALSE(root->gui_is_dragging());
 
@@ -1131,7 +1145,7 @@ TEST_CASE("[SceneTree][Viewport] Controls and InputEvent handling") {
 				CHECK((StringName)node_d->drag_data == SNAME("Drag Data"));
 			}
 
-			SUBCASE("[Viewport][GuiInputEvent] Perform unsuccessful drop on Control.") {
+			SUBCASE("[Viewport][GuiInputEvent][DnD] Perform unsuccessful drop on Control.") {
 				SEND_GUI_MOUSE_BUTTON_EVENT(on_a, MouseButton::LEFT, MouseButtonMask::LEFT, Key::NONE);
 				CHECK_FALSE(root->gui_is_dragging());
 
@@ -1157,7 +1171,7 @@ TEST_CASE("[SceneTree][Viewport] Controls and InputEvent handling") {
 				CHECK_FALSE(root->gui_is_drag_successful());
 			}
 
-			SUBCASE("[Viewport][GuiInputEvent] Perform unsuccessful drop on No-Control.") {
+			SUBCASE("[Viewport][GuiInputEvent][DnD] Perform unsuccessful drop on No-Control.") {
 				SEND_GUI_MOUSE_BUTTON_EVENT(on_a, MouseButton::LEFT, MouseButtonMask::LEFT, Key::NONE);
 				CHECK_FALSE(root->gui_is_dragging());
 
@@ -1171,7 +1185,7 @@ TEST_CASE("[SceneTree][Viewport] Controls and InputEvent handling") {
 
 				// Move away from Controls.
 				SEND_GUI_MOUSE_MOTION_EVENT(on_background, MouseButtonMask::LEFT, Key::NONE);
-				CHECK(DS->get_cursor_shape() == DisplayServer::CURSOR_ARROW); // This could also be CURSOR_FORBIDDEN.
+				CHECK(DS->get_cursor_shape() == DisplayServer::CURSOR_ARROW);
 
 				CHECK(root->gui_is_dragging());
 				SEND_GUI_MOUSE_BUTTON_RELEASED_EVENT(on_background, MouseButton::LEFT, MouseButtonMask::NONE, Key::NONE);
@@ -1179,7 +1193,7 @@ TEST_CASE("[SceneTree][Viewport] Controls and InputEvent handling") {
 				CHECK_FALSE(root->gui_is_drag_successful());
 			}
 
-			SUBCASE("[Viewport][GuiInputEvent] Perform unsuccessful drop outside of window.") {
+			SUBCASE("[Viewport][GuiInputEvent][DnD] Perform unsuccessful drop outside of window.") {
 				SEND_GUI_MOUSE_BUTTON_EVENT(on_a, MouseButton::LEFT, MouseButtonMask::LEFT, Key::NONE);
 				CHECK_FALSE(root->gui_is_dragging());
 
@@ -1192,7 +1206,6 @@ TEST_CASE("[SceneTree][Viewport] Controls and InputEvent handling") {
 
 				// Move outside of window.
 				SEND_GUI_MOUSE_MOTION_EVENT(on_outside, MouseButtonMask::LEFT, Key::NONE);
-				CHECK(DS->get_cursor_shape() == DisplayServer::CURSOR_ARROW);
 				CHECK(root->gui_is_dragging());
 
 				SEND_GUI_MOUSE_BUTTON_RELEASED_EVENT(on_outside, MouseButton::LEFT, MouseButtonMask::NONE, Key::NONE);
@@ -1200,7 +1213,7 @@ TEST_CASE("[SceneTree][Viewport] Controls and InputEvent handling") {
 				CHECK_FALSE(root->gui_is_drag_successful());
 			}
 
-			SUBCASE("[Viewport][GuiInputEvent] Drag and Drop doesn't work with other Mouse Buttons than LMB.") {
+			SUBCASE("[Viewport][GuiInputEvent][DnD] Drag and Drop doesn't work with other Mouse Buttons than LMB.") {
 				SEND_GUI_MOUSE_BUTTON_EVENT(on_a, MouseButton::MIDDLE, MouseButtonMask::MIDDLE, Key::NONE);
 				CHECK_FALSE(root->gui_is_dragging());
 
@@ -1209,7 +1222,7 @@ TEST_CASE("[SceneTree][Viewport] Controls and InputEvent handling") {
 				SEND_GUI_MOUSE_BUTTON_RELEASED_EVENT(on_a, MouseButton::MIDDLE, MouseButtonMask::NONE, Key::NONE);
 			}
 
-			SUBCASE("[Viewport][GuiInputEvent] Drag and Drop parent propagation.") {
+			SUBCASE("[Viewport][GuiInputEvent][DnD] Drag and Drop parent propagation.") {
 				Node2D *node_aa = memnew(Node2D);
 				Control *node_aaa = memnew(Control);
 				Node2D *node_dd = memnew(Node2D);
@@ -1318,7 +1331,7 @@ TEST_CASE("[SceneTree][Viewport] Controls and InputEvent handling") {
 				memdelete(node_aa);
 			}
 
-			SUBCASE("[Viewport][GuiInputEvent] Force Drag and Drop.") {
+			SUBCASE("[Viewport][GuiInputEvent][DnD] Force Drag and Drop.") {
 				SEND_GUI_MOUSE_MOTION_EVENT(on_background, MouseButtonMask::NONE, Key::NONE);
 				CHECK_FALSE(root->gui_is_dragging());
 				node_a->force_drag(SNAME("Drag Data"), nullptr);
@@ -1339,6 +1352,111 @@ TEST_CASE("[SceneTree][Viewport] Controls and InputEvent handling") {
 				SEND_GUI_MOUSE_BUTTON_RELEASED_EVENT(on_d, MouseButton::LEFT, MouseButtonMask::NONE, Key::NONE);
 			}
 		}
+
+		SUBCASE("[Viewport][GuiInputEvent][DnD] Drag to a different Viewport.") {
+			SubViewportContainer *svc = memnew(SubViewportContainer);
+			svc->set_size(Size2(100, 100));
+			svc->set_position(Point2(200, 50));
+			root->add_child(svc);
+
+			SubViewport *sv = memnew(SubViewport);
+			sv->set_embedding_subwindows(true);
+			sv->set_size(Size2i(100, 100));
+			svc->add_child(sv);
+
+			DragStart *sv_a = memnew(DragStart);
+			sv_a->set_position(Point2(10, 10));
+			sv_a->set_size(Size2(10, 10));
+			sv->add_child(sv_a);
+			Point2i on_sva = Point2i(215, 65);
+
+			DragTarget *sv_b = memnew(DragTarget);
+			sv_b->set_position(Point2(30, 30));
+			sv_b->set_size(Size2(20, 20));
+			sv->add_child(sv_b);
+			Point2i on_svb = Point2i(235, 85);
+
+			Window *ew = memnew(Window);
+			ew->set_position(Point2(50, 200));
+			ew->set_size(Size2(100, 100));
+			root->add_child(ew);
+
+			DragStart *ew_a = memnew(DragStart);
+			ew_a->set_position(Point2(10, 10));
+			ew_a->set_size(Size2(10, 10));
+			ew->add_child(ew_a);
+			Point2i on_ewa = Point2i(65, 215);
+
+			DragTarget *ew_b = memnew(DragTarget);
+			ew_b->set_position(Point2(30, 30));
+			ew_b->set_size(Size2(20, 20));
+			ew->add_child(ew_b);
+			Point2i on_ewb = Point2i(85, 235);
+
+			SUBCASE("[Viewport][GuiInputEvent][DnD] Drag to SubViewport") {
+				sv_b->valid_drop = false;
+				SEND_GUI_MOUSE_BUTTON_EVENT(on_a, MouseButton::LEFT, MouseButtonMask::LEFT, Key::NONE);
+				SEND_GUI_MOUSE_MOTION_EVENT(on_a + Point2i(min_grab_movement, 0), MouseButtonMask::LEFT, Key::NONE);
+				CHECK(root->gui_is_dragging());
+				CHECK(sv_b->during_drag);
+				SEND_GUI_MOUSE_MOTION_EVENT(on_svb, MouseButtonMask::LEFT, Key::NONE);
+				CHECK(DS->get_cursor_shape() == DisplayServer::CURSOR_CAN_DROP);
+
+				SEND_GUI_MOUSE_BUTTON_RELEASED_EVENT(on_svb, MouseButton::LEFT, MouseButtonMask::NONE, Key::NONE);
+				CHECK(sv_b->valid_drop);
+				CHECK(!sv_b->during_drag);
+			}
+
+			SUBCASE("[Viewport][GuiInputEvent][DnD] Drag from SubViewport") {
+				node_d->valid_drop = false;
+				SEND_GUI_MOUSE_BUTTON_EVENT(on_sva, MouseButton::LEFT, MouseButtonMask::LEFT, Key::NONE);
+				SEND_GUI_MOUSE_MOTION_EVENT(on_sva + Point2i(min_grab_movement, 0), MouseButtonMask::LEFT, Key::NONE);
+				CHECK(sv->gui_is_dragging());
+				CHECK(node_d->during_drag);
+				SEND_GUI_MOUSE_MOTION_EVENT(on_d, MouseButtonMask::LEFT, Key::NONE);
+				CHECK(DS->get_cursor_shape() == DisplayServer::CURSOR_CAN_DROP);
+
+				SEND_GUI_MOUSE_BUTTON_RELEASED_EVENT(on_d, MouseButton::LEFT, MouseButtonMask::NONE, Key::NONE);
+				CHECK(node_d->valid_drop);
+				CHECK(!node_d->during_drag);
+			}
+
+			SUBCASE("[Viewport][GuiInputEvent][DnD] Drag to embedded Window") {
+				ew_b->valid_drop = false;
+				SEND_GUI_MOUSE_BUTTON_EVENT(on_a, MouseButton::LEFT, MouseButtonMask::LEFT, Key::NONE);
+				SEND_GUI_MOUSE_MOTION_EVENT(on_a + Point2i(min_grab_movement, 0), MouseButtonMask::LEFT, Key::NONE);
+				CHECK(root->gui_is_dragging());
+				CHECK(ew_b->during_drag);
+				SEND_GUI_MOUSE_MOTION_EVENT(on_ewb, MouseButtonMask::LEFT, Key::NONE);
+				CHECK(DS->get_cursor_shape() == DisplayServer::CURSOR_CAN_DROP);
+
+				SEND_GUI_MOUSE_BUTTON_RELEASED_EVENT(on_ewb, MouseButton::LEFT, MouseButtonMask::NONE, Key::NONE);
+				CHECK(ew_b->valid_drop);
+				CHECK(!ew_b->during_drag);
+			}
+
+			SUBCASE("[Viewport][GuiInputEvent][DnD] Drag from embedded Window") {
+				node_d->valid_drop = false;
+				SEND_GUI_MOUSE_BUTTON_EVENT(on_ewa, MouseButton::LEFT, MouseButtonMask::LEFT, Key::NONE);
+				SEND_GUI_MOUSE_MOTION_EVENT(on_ewa + Point2i(min_grab_movement, 0), MouseButtonMask::LEFT, Key::NONE);
+				CHECK(ew->gui_is_dragging());
+				CHECK(node_d->during_drag);
+				SEND_GUI_MOUSE_MOTION_EVENT(on_d, MouseButtonMask::LEFT, Key::NONE);
+				CHECK(DS->get_cursor_shape() == DisplayServer::CURSOR_CAN_DROP);
+
+				SEND_GUI_MOUSE_BUTTON_RELEASED_EVENT(on_d, MouseButton::LEFT, MouseButtonMask::NONE, Key::NONE);
+				CHECK(node_d->valid_drop);
+				CHECK(!node_d->during_drag);
+			}
+
+			memdelete(ew_a);
+			memdelete(ew_b);
+			memdelete(ew);
+			memdelete(sv_a);
+			memdelete(sv_b);
+			memdelete(sv);
+			memdelete(svc);
+		}
 	}
 
 	memdelete(node_j);