http://bugs.winehq.org/show_bug.cgi?id=26185
--- Comment #5 from Austin Lund austin.lund@gmail.com 2011-03-01 16:10:05 CST --- I think I've figured out that this has to do with the program using hooks to respond to mouse messages instead of a more orthodox approach (which I assumed when setting the title of the bug originally).
I've been able to confuse wine's WH_MSGFILTER reporting for things like moving a message box (the MSG->pt variable isn't updated) but haven't been able to find anything wrong with the WM_LBUTTONDOWN messages.
I think this may also be related to bug 25282.