For content author, I think users did not see this bug, but it was discovered during development.
Upon (linux) guest panic, a spice-client connected to the VM caused client's keyboard leds to blink and capslock to turn on/off continuously, in non-spice-client windows too.
Blocking keyboard modifier events when
spice-client windows is not in-focus.
That means when the user focuses out of spice-client window (focus on a different window), blinks stop blinking and caps-lock does not turn on/off.