Fix mouse position with screen transform

When a Viewport is not directly attached to the screen, the
function `Viewport::get_mouse_position` can't rely on
`get_screen_transform`, because that function is ambiguous in
these situations.
In these cases it is necessary to use the mouse position from
the most recent mouse IputEvent.

scene/main/viewport.cpp

@@ -1351,7 +1351,11 @@ Ref<InputEvent> Viewport::_make_input_local(const Ref<InputEvent> &ev) {
 
 Vector2 Viewport::get_mouse_position() const {
 	ERR_READ_THREAD_GUARD_V(Vector2());
-	if (DisplayServer::get_singleton()->has_feature(DisplayServer::FEATURE_MOUSE)) {
+	if (!is_directly_attached_to_screen()) {
+		// Rely on the most recent mouse coordinate from an InputEventMouse in push_input.
+		// In this case get_screen_transform is not applicable, because it is ambiguous.
+		return gui.last_mouse_pos;
+	} else if (DisplayServer::get_singleton()->has_feature(DisplayServer::FEATURE_MOUSE)) {
 		return get_screen_transform_internal(true).affine_inverse().xform(DisplayServer::get_singleton()->mouse_get_position());
 	} else {
 		// Fallback to Input for getting mouse position in case of emulated mouse.
@@ -4595,6 +4599,11 @@ Transform2D SubViewport::get_popup_base_transform() const {
 	return c->get_screen_transform() * container_transform * get_final_transform();
 }
 
+bool SubViewport::is_directly_attached_to_screen() const {
+	// SubViewports, that are used as Textures are not considered to be directly attached to screen.
+	return Object::cast_to<SubViewportContainer>(get_parent()) && get_parent()->get_viewport() && get_parent()->get_viewport()->is_directly_attached_to_screen();
+}
+
 void SubViewport::_notification(int p_what) {
 	ERR_MAIN_THREAD_GUARD;
 	switch (p_what) {

scene/main/viewport.h

@@ -669,6 +669,7 @@ public:
 	Transform2D get_screen_transform() const;
 	virtual Transform2D get_screen_transform_internal(bool p_absolute_position = false) const;
 	virtual Transform2D get_popup_base_transform() const { return Transform2D(); }
+	virtual bool is_directly_attached_to_screen() const { return false; };
 
 #ifndef _3D_DISABLED
 	bool use_xr = false;
@@ -800,6 +801,7 @@ public:
 
 	virtual Transform2D get_screen_transform_internal(bool p_absolute_position = false) const override;
 	virtual Transform2D get_popup_base_transform() const override;
+	virtual bool is_directly_attached_to_screen() const override;
 
 	void _validate_property(PropertyInfo &p_property) const;
 	SubViewport();

scene/main/window.cpp

@@ -2475,6 +2475,14 @@ Transform2D Window::get_popup_base_transform() const {
 	return popup_base_transform;
 }
 
+bool Window::is_directly_attached_to_screen() const {
+	if (get_embedder()) {
+		return get_embedder()->is_directly_attached_to_screen();
+	}
+	// Distinguish between the case that this is a native Window and not inside the tree.
+	return is_inside_tree();
+}
+
 void Window::_bind_methods() {
 	ClassDB::bind_method(D_METHOD("set_title", "title"), &Window::set_title);
 	ClassDB::bind_method(D_METHOD("get_title"), &Window::get_title);

scene/main/window.h

@@ -401,6 +401,7 @@ public:
 	virtual Transform2D get_final_transform() const override;
 	virtual Transform2D get_screen_transform_internal(bool p_absolute_position = false) const override;
 	virtual Transform2D get_popup_base_transform() const override;
+	virtual bool is_directly_attached_to_screen() const override;
 
 	Rect2i get_parent_rect() const;
 	virtual DisplayServer::WindowID get_window_id() const override;