One way that the typically extremely frustrating use of GWT Designer could be significant improved would be to have greater intelligence in the way that mouse clicks are handled for widget selection.
A common use case for e.g. SmartGWT are using tabsets with multiple tabs. Then it's certainly possible to, in turn, have tabsets within a given tab hierarchically.
This means that there are guaranteed to be overlapping widgets in the same physical screen space - widgets that will NEVER be displayed simultaneously in a running app.
Mouse selection should be smart enough to recognize the current working context (a given tab) and NOT just randomly pick some widget within ANOTHER tab. It should be necessary to select a different tab in the Structure view in order to change the working context. Moreover, ONLY widgets (including their border-outlines) in the same tab should be visible at any given time.
This would hugely improve the user experience.