Fix UI element passing above the header when scrolling

[aurelticot]

The search result popover may be passing above the header.
Ppopovers have the same z-index as the header, so their positions in the DOM makes them above the header.

We could set a higher z-index for the header but the network switcher dropdown menu (using a popover) becomes underneath the header and apparently can't change it's z-index (can for the content but not its container which is what matters). This is the way to explore though.