Wow, so many answers in so short time. Thank you all.
To clarify, the moving squares in the video are sliders and / or radiobuttons, and they are moved in runtime. Attached is a patch demonstrating the idea, using [toxy/tot].
As it seems, [iemguts/receivecanvas] could do this job indeed. It captures cursor position relative to the object's containing window, or relative to a parent if so desired. Phew, this will save my projects! I never knew about the existence of [iemguts/receivecanvas]. It has no helpfile. I'd be happy to make one though. Hopefully the iem libs will stay a bit longer than toxy, though they're now categorized as 'unsupported, included libraries'.
Jonathan, can you explain how a larger area of a polygon can be made reactive in runtime? Do you have a demo patch? I'm still interested in the data structure route, since I use them for parameter values anyway, and it is a bit underestimated aspect of Pure Data in my view.
Katja