The underlying abstract YaST UI library (libyui) already had most of what it took to make this happen: An abstract concept of properties for each widget class including setting, getting and enumerating those properties.

What I had to add was the table widget for the YDialogSpy window (of course) and to add some more fields of the YWidget base class of all YaST widgets to those properties (VStretch, HStretch, VWeight, HWeight). This has the added benefit that they can now also queried and changed as properties from YCP code.

BTW the funny thing about this YDialogSpy is that you can apply it onto itself: You can start another YDialogSpy from the YDialogSpy if you hit Ctrl-Shift-Alt-Y there. That’s how I colorized the widgets in the screen shots above.

Further reading
http://en.opensuse.org/YaST/Development/Misc/YDialogSpy