Thanks. v2.61 doesn't have the problem.
In v2.66.01, it happens when window is first displayed immediately before the last 9 results are displayed. So, to answer your question, it's when displaying normal results. It looks like the sequence of events is, 1-window is displayed, 2-window resizes (at which point the content area is black), 3-content area is rendered (with text, icons, and white background). It happens very quickly, but the black to white change produces a noticeable "flickering" effect. Hopefully that is helpful.
One additional note. After the initial results are displayed, it doesn't happen as the results change.