Send mouse-enter notification, when viewport receives a mouse motion event

Ensure, that a `SubViewport` receives a `NOTIFICATION_VP_MOUSE_ENTER`,
when an `InputEventMouseMotion` is sent to a viewport, that currently
doesn't have the mouse inside.

In the following case the changed code makes a difference:
By default the viewport has the state, that the mouse is not over
the viewport.
When the user pushes an `InputEventMouseMotion` to a `SubViewport`,
then because of the default state, the Viewport assumes, that the mouse
is not over itself.
With this change, the Viewport automatically switches its state.

Make necessary adjustments to unit-tests.

Author: Sauermann

scene/main/viewport.cpp

@@ -646,6 +646,7 @@ void Viewport::_notification(int p_what) {
 
 		case NOTIFICATION_VP_MOUSE_EXIT: {
 			gui.mouse_in_viewport = false;
+			gui.drag_mouse_over = nullptr;
 			_drop_physics_mouseover();
 			// When the mouse exits the viewport, we don't want to end
 			// mouse_focus, because, for example, we want to continue
@@ -2967,6 +2968,12 @@ void Viewport::_update_mouse_over() {
 		return;
 	}
 
+	if (!gui.mouse_in_viewport) {
+		// This is a hack, that makes pushing mouse motion events to SubViewports more user-friendly.
+		// Without this, a user needs to take care of sending mouse-enter/exit notifications to SubViewports.
+		notification(NOTIFICATION_VP_MOUSE_ENTER);
+	}
+
 	if (get_tree()->get_root()->is_embedding_subwindows() || is_sub_viewport()) {
 		// Use embedder logic for calculating mouse position.
 		_update_mouse_over(gui.last_mouse_pos);

tests/display_server_mock.h

@@ -113,15 +113,22 @@ class DisplayServerMock : public DisplayServerHeadless {
 	void simulate_event(Ref<InputEvent> p_event) {
 		Ref<InputEvent> event = p_event;
 		Ref<InputEventMouse> me = p_event;
+		bool parse_event = true;
 		if (me.is_valid()) {
 			Ref<InputEventMouseMotion> mm = p_event;
 			if (mm.is_valid()) {
 				mm->set_relative(mm->get_position() - mouse_position);
 				event = mm;
 			}
 			_set_mouse_position(me->get_position());
+			if (mm.is_valid() && !window_over) {
+				parse_event = false;
+			}
+		}
+
+		if (parse_event) {
+			Input::get_singleton()->parse_input_event(event);
 		}
-		Input::get_singleton()->parse_input_event(event);
 	}
 
 	// Returns the current cursor shape.

tests/scene/test_viewport.h

@@ -1192,7 +1192,6 @@ 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);