Here is the bug description from Matt:

... in our app, the program would switch from our MIDAS editor to the Display editor when I clicked one of our buttons on the top of the editor.
I think its because those little widgets that hover over the MITK Display, where you have 3 little icons (a plus, a square and a tool icon) are getting in the way.
So, when the editor switches, these icons don't know that they are meant to be invisible and disabled, and it just happens to be in the same place as my button, but behind it, but it steals the mouse click. At it only appears to happen on a Mac!