Twenty fourteen fixed navigation in internet explorer 11

Hello,

The horizontal menu bar in twenty fourteen is fixed (or sticky). When you scroll down, it stays on top of the screen. This works in any browser I tested. Only when you add a header, in Internet Explorer 11, the menu bar loses this property and disappears when you scroll down.

Any ideas on how the menu bar can be “fixed” in Internet Explorer 11?

The site I’m working on is https://www.instrumentgravures.be.

Thanks