I also attempted to wait for 0 unit of time (second or microsecond) to no avail. The null delay is probably detected somewhere and optimised out.

BrowserUk says yield is a mistake (like sleep 0), so sleep more, like 1/2 (or less) of the minimum time period that you believe it will take, like 33 milliseconds -- seems to hold true on win32, good rule of thumb :)