What is your autofit setting? You can find this by clicking View->Autosize while in full screen mode.
The screen shot shows that you have many tabs open. Is there a specific order in which you go through the tabs to reproduce the problem? Does it happen if you have just, say, two running VMs open?
The autofit setting is on "Center Guest".
Found a way to reproduce it:
1. Windows 7 x64
2. Windows Server 2008 R2
1) Powered on VM in tab 1 in windowed mode
2) Entered fullscreen with tab 1 active
3) Switched to tab 2, pressed 'Power on this virtual machine'
4) waited a few seconds (VM in tab 2 was still booting) switched backed to tab 1
5) VM in tab 1 was still displayed correctly
6) switched to tab 2, guest was displayed on small portion with black borders
Thanks for the info! I can reproduce the problem, so I'll enter a bug in our tracker.