border radius conflicts with primary menu

In my twentyfourteen child theme i have made this alteration:

div.header-main {
border: 4px solid #000;
border-radius: 25px 25px 0px 0px;
background-color: #000;
}

.search-toggle {
border: 2px solid;
border-radius: 25px;
}

#content-sidebar {
background-color: #F1F1F1;
border: 4px solid #F1F1F1;
border-radius: 0px 0px 0px 25px;
}

Now i found that the primary menu does not stay on top of the page when you scroll down (like it did before) but it disapears just like any regular site.

It looks like the border-radius causes this problem.

Is there an alternative for border radius with the same result or do i have to let go of the idea?