interactive buttons inside treeview headings

[Stefan Kost <ensonic hora-obscura de>]

hi list,

I've added a couple of (toggle)-buttons and a label in a box into each
of my treeview headers. They are shown properly. I can also modify them
programatically (toggle).
My problem is, that I can't click them. Honestly I have no idea how to
debug this. Any idea?

Thanks a lot.
  Stefan