Once added, the panel now displays an Aqua colored progress bar.
I haven't tested this, but I think you'd really want to override
canBecomeKeyWindow, not isKeyWindow. The former means your window can
become the key view when the user designates it as such (by clicking
or using Cmd-~ or whatever). If you override the latter, you're
effectively lying to everyone about your window.
So, in general, you want to override canBecomeMain or canBecomeKey,
not isMainWindow or isKeyWindow.