> By looking at these functions you could derive a new one
 > that calculates mode line heights in a similar way and peruse the return
 > values of that function in 'window-text-pixel-size' in the hope that
 > they won't change with the next redisplay.  The 'mode-lines' argument of
 > 'window-text-pixel-size' should then accept a value like 'update' to
 > reflect this special use case.  It might be worth the experience.

Or try the attached patch on master.  It seems to fix the issue here.

martin