Redrawing of group boundary on scroll sometimes wrong

Sometimes when scrolling with groups the top and bottom group boundary lines are extended to the right - see screenshot.

To reproduce: open uniref50.fa, create two groups and hide columns as shown in the screenshot (I suspect that only hiding the columns between the groups is necessary). Scroll right and left a few times. Varying the speed and position of the scroll can result in the top and bottom of the group boundary of the first group being extended to the right.