Comment 6 for bug 55739

No, for me it always happens when I'm actively using the computer. In fact, it mostly seems to happen following some rapid mouse clicking.

I bet focus-follows-pointer is a necessary condition to trigger the bug. That would explain why lots of other people haven't noticed it. f-f-p is probably a minority choice, and there are probably other necessary conditions too which we don't know about which further reduce the prevelance of this bug (e.g., my graphics driver is mga).