Sander,
I am at my main Windows development machine now, and I can confirm this bug.
It happens sporadically just when you open a tool window; in no other circumstance, at least in all my tests. It never happens when you click on an image window, either on the image or on any part of the image's frame, including the title bar, the view selector tray, view selectors and other decorations (if that happened, then the bug would be really serious).
What exactly do you refer to when you say 'when clicking on a tab'?
Interestingly, this seems to be a Windows-only bug. It never happens on X11 and I can't reproduce it on Mac OS X x86.