I play on a PC running windows 7 and have run into this bug on 4 different computers. This bug has existed since SK was put on Steam, I just never bothered to report it.
Whenever I tab out then back in to Spiral Knights while in full screen, it does not go back into full screen correctly. This is because I have my taskbar on the left side of my monitor instead of the bottom. Instead, this happens.
To fix the problem, I have to go into options, turn off full screen, and then turn it back on. It would be much appreciated if this bug could be sorted out sometime.
It's happening to me sometimes when I launch SK, but I'm running the standalone client - not bound to Steam. I'm running Win7 (64-bit), 64-bit JVM 1.7, and taskbar is at the bottom of my screen.
I do not need to toggle the fullscreen option to fix it; all I need is just to click on SK's button on taskbar once, to properly set the focus.
Alt-tabbing does not cause this problem for me.