Fix position and size of MenuButton popup

Use the same logic as in OptionButton::show_popup

scene/gui/menu_button.cpp

@@ -97,15 +97,14 @@ void MenuButton::show_popup() {
 	}
 
 	emit_signal(SNAME("about_to_popup"));
-	Size2 size = get_size() * get_viewport()->get_canvas_transform().get_scale();
-
-	popup->set_size(Size2(size.width, 0));
-	Point2 gp = get_screen_position();
-	gp.y += size.y;
+	Rect2 rect = get_screen_rect();
+	rect.position.y += rect.size.height;
+	rect.size.height = 0;
+	popup->set_size(rect.size);
 	if (is_layout_rtl()) {
-		gp.x += size.width - popup->get_size().width;
+		rect.position.x += rect.size.width - popup->get_size().width;
 	}
-	popup->set_position(gp);
+	popup->set_position(rect.position);
 
 	// If not triggered by the mouse, start the popup with its first enabled item focused.
 	if (!_was_pressed_by_mouse()) {