Full screen menu help needed

Hey all -

Was hoping someone could help me troubleshoot. I’m trying to create a full screen menu while menu is open. While it appears to work just fine when I’m in Designer Mode, when I preview or publish, the menu gets cut off at the top. Or, rather, the menu begins to drop open down just below my header. Would someone be willing to take a peek at this and see if its something obvious? I know there are other posts seem similar to this, but I think I’m having a different issue. Thanks!