I noticed that the thumbnails on the task bar disappear when the windows ones wouldn't. I tracked the problem down to the space between the task bar and the thumbnail box. If your mouse takes to long to cross the gap the thumbnail will close. The real task bar does not have this problem.

So perhaps a delay needs to be added before the thumbnail disappears. Or tracking of the mouse around the thumbnail which is what windows appears to do, since you can have your mouse leave the thumbnail and when it's a certain distance away it closes. There also seems to be a set delay time with the windows thumbnails also.