The replay-progress check polled the overlay from the test every 100ms. A replay that starts and finishes between two polls is never sampled, and the test then reports that no progress was shown at all -- the source of the intermittent "no progress was shown while the history loaded" failure. Record the samples from a MutationObserver installed before the page's own scripts run, so a fast replay is observed rather than missed. Signed-off-by: Stan Grams <sjg@haxx.space>