Windows were composited from different times

[jnm2]

There was an example of typing and using the mouse simultaneously in a modal window and the window behind it, and the window behind it was showing the wrong series of images.

For one idea, double-check whether there could be a failure to grab images from deactivated windows or owners of modal windows, and make sure we don't confuse lack of images with zero time passing. (There's a circular buffer.)