This was broken in Chrome since a couple of releases ago, let me add the upstream bug later. The problem is that the width of the window changes at the end of the test and that is picked up as a last visual change.
I think there's a workaround to set the initial window size using Chrome command line (so it is the same as we later set). I did a quick try and the "blink" at the end is gone but something else is picked up as last change, let me try it some more.