Don't you just hate it when kernel messages pollute your display in the form of those annoying black rectangles overlaid above whatever you're working on? This is a known bug in rio(1); for further detail, see Rob Pike's message to this effect.

To work around this problem, open a new terminal window and do:

cat /dev/kprint

This works because, according to the kprint discussion in cons(3), "if (the) console is a graphics screen, the data is sent either to the display or to kprint, but not both."

Another more permanent solution to the problem is to put

cat /dev/kprint >/dev/null >[2=1]

somewhere in termrc (see cpurc(8)).