 <p>It seems to be related to a sort of "IE9 z-index" bug. I found this: <a href="http://icreatestuff.co.uk/blog/article/ie9-z-index-stacking-problem-or-something-stranger" rel="nofollow">http://icreatestuff.co.uk/blog/article/ie9-z-index-stacking-problem-or-something-stranger</a></p> <p>and added a transparent 1x1 "fixer.png" to the navbar. Now it works immediately in IE9 without resizing. Glad, that I am using Firefox most of the times. :-)</p>