I think that’s a custom header.
It’s tricky because the popup is a child of .reply-control. It has a z-index of 999. The header drop-down menus inherit a z-index of 1000 from the header. If you reduce the header’s z-index then the dropdown menu shows up behind the composer.
One solution is .custom-header { position: relative; z-index: 1000; } That way at least the header and the custom-header are both covering the popup.