3.7.4 Restoring Default Mouse Handling
As soon as you define #handlesMouseDown: in your Morph subclass, you lose the default behavior where a morph is picked up when clicked on.
To restore this behavior, compile the following two methods in your morph class:
mouseDown: anEvent
anEvent hand waitForClicksOrDrag: self event: anEvent.
startDrag: anEvent
anEvent hand grabMorph: self.