Possible enhancement: a symbol in the box below each for the various interfaces they support (mouse, keyboard, focus, etc).
It sounded like a great idea, so I spent some time working on it today. The linked image above is the initial version this enhancement. While looking through the GWT source I found some redundancies in interface usage, and decided to not include the redundancies in the map. One example of this is the TextBoxBase class, which implements the SourcesKeyboardEvents interface. The duplication is that TextBoxBase extends FocusWidget, which in turn implements HasFocus, and HasFocus extends SourcesKeyboardEvents. Some of the redundancies were not obvious until I went to chart them on the map, and could visually see the redundancies.
So far the following interfaces appear on the map: SourcesChangeEvents, HasFocus, HasText, HasHTML, EventListener, SourcesClickEvents, SourcesLoadEvents, SourcesMouseEvents, SourcesKeyboardEvents (by inheritance), and SourcesFocusEvents (by inheritance). There isn't a map legend yet, but I hope it won't be too difficult to figure out.
And for the sake of completeness, the following interfaces still need to be added: ClickListener, HasAlignment, HasHorizontalAlignment, HasVerticalAlignment, HasWidgets, HasWordWrap, MouseListener, PopupListener, SourcesPopupEvents, SourcesTabEvents, SourcesTableEvents, SourcesTreeEvents, and TabListener. Any other interface not listed here was either found to not be used by any classes, or can be traced back through some child interface.
No promises, but I hope to finish it in the next couple of weeks. Let me know if you have any other ideas for enhancing the map.