Did you ever resolve this? I'd be interested in the solution. :) The difficult we do right away... ...the impossible takes slightly longer.

Found it! There is an intermediate class which inherited from CTreeCtrl. It was originally set up to support a CView, and forwarded the LButtonDown event to its parent. Well, now that the parent is a dockable pane, the behaviour changed a lot! Removing that handler got lots of stuff working!