A relatively late correction to refresh handling for El Capitan
(commit 9bf18505d5) causes a canvas to be refreshed too often
in some cases. Delay the refresh to restore the old timing
without the old bug.
Ensure that a blink timer for a canvas stops running
if the canvas becomes hidden. Otherwise, though a race
condition, it's possible for a GUI program to never
terminate if a "focus-out" event gets lost.
