and figured I would do my instanceof check after the assignment. This will compile, but the line:
if(event.getSource() instanceof Object)
will throw ClassCastException! It complains that TreeSelectionModel cannot be cast to Component
The complaint is actually with line 57 in SelectionChangedEvent.java. That line is trying to downcast an Object (returned by GwtEvent.getSource()) as a Component, and TreeSelectionModel is not in the Component hierarchy.
My workaround is to have a handle to the Tree available, and use the onSelectionChanged method simply to note when the selection has changed. I have to ignore the SelectionChangedEvent instance that's passed in, because I can't do much with it.
However, it looks like SelectionChangedEvent.getSource() should be returning an Object rather than a Component... (unless I'm missing something)