To reproduce run the demo:
Unpin the 'Server Explorer'.
Moving your mouse over 'Server Explorer' tab header causes the window to fly out. On moving the mouse out of the tool window it then closes.
This behaviour however stops working once you click on the 'Server Explorer' tab header. Once you have clicked it's impossible for the window to close itself when the mouse is moved out of the tool window.
This issue has been a thorn for quite a while and I've noticed other users have also raised PITS suggesting clicking on the tab header should close the tool window. Either way we either need this behaviour fixed or the click on tab header closes fix implemented.