This is the intended behavior; it’s meant to handle the case of clickable objects on the desktop, which behave in this way: when you click on a Windows GUI button, for instance, and drag the mouse off of the button while the mouse button is still held, you cannot trigger any other buttons you drag off. The mouse is “captured” or still considered held by the original button, and the MouseWatcher follows this model.
If you require different behavior, you can always write a task to implement the precise behavior you require. 
David