I've had this problem for a long time now also. It doesn't happen till I've played for a while, and a reloadui fixes it.. Havn't tested it much lately, since I've just assigned hotkeys to my closetgnomes outfits :)
I was still having the issue after the update. I seemed to narrow it down to the prat fubar-plugin. If the bar was forced to dock at the top with the prat plugin it would cause nothing to be clickable except in a tiny sliver just below the bar (fubar config itself could be clicked).
Behavior is not the same when the bar is not docked to the top or prat-fubar is removed from the bar (docked to the minimap for example). It works properly in those scenarios.
Other odd thing I noticed with the prat-fubar is that it would not let me drag and drop to any other fubars or among the same fubar.