Checks the field now, and on the next few frames (see framesOf). A
check measures the field only if anything the floor reads has changed
since it last measured, and changes nothing on the page where nothing
has. The checks the floor makes on its own, on a frame or on focus, log
what they throw; this one leaves it to its caller.
Stops checking, and puts back what the floor replaced, if anything.
One field's floor, for as long as the field is drawn.