I'm not seeing that on my Win7 box with either v4.1.3 or 4.2.
Is SQL Developer maximized or not?
Are you running anything in SQLDev, maybe backgrounded a task?
Do you have a report set to auto-refresh?
Thanks for the response Jeff.
SQLD max or min makes no difference.
It appears to be related to having Win7 set to 'focus follows mouse'. Interestingly SQL Developer behaves completely differently to my other common applications (Explorer, Chrome, Vim, Office Apps etc.) in that placing the mouse over SQL Developer does nothing - I still have to click in the window in order to move the focus.