.top-bar { z-index: 5050; } versus #overlay-header { z-index: 5052; } so the overlay should win. Strangely enough disabling the top bar z-index fixes the issue. Another fix is to set the overlay background alpha to 1 instead of 0.95

So maybe a combination of the z-index and alpha is responsible for this?

Using Firefox 67.0.1 on Windows 10.