1) Sorry, the last solution you posted slipped my eyes, before I posted my solution.
After some considerations I am going for your quickfix solution, since that gives me what I need without having to override a whole lot.
2) My panel hasn't got a header so I can't use the header. But everything works fine by locking the dd, and setting the cursor onafterrender as you suggested, so I am going to stick with that!
Thank you a bunch for the quickfix!
P.S. I would still very much like to see the config option resizable implemented in border layout some day, there are perfectly sane usecases where you'd want a collapsible but not resizable panel.