When dragging a window with a native title bar and a menu, a menu item will highlight when the menu moves over the point where the title bar was clicked during a drag operation. For example:
Click and hold in the title bar, then drag up by just one pixel:
The problem seems to happen after you click a top level menu and release. It might not happen without first pulling a menu down. It also seems that the highlighting may be tied to the location, in global screen coordinates where you initially clicked the menu. For example, click the File menu then click again to dismiss it. Now drag the title. Result: When the File top level menu contains the initial click in global coordinates, it will highlight during the window drag operation.
Obviously - menus should not highlight during a window drag.