It’s really puzzling. 
I’ve not been able to reproduce this yet, but what I’ve done for now is made sure that the composer can’t be covered up by the navigation anymore, so hopefully that should make the bug less severe. (This change will go live tonight.)
If anyone has any other pointers about how to recreate this, it would be very helpful. (Also whether it occurs in different browsers on the same system, and which.)
To be clear, the New Post panel should never be displayed over the top of or under the navigation.