When in desktop mode, if a user goes prone using a custom sitting animation, and then opens the quick menu and enters the Avatars, Worlds, or any of the menus, the menu will open at the height as though the player was standing, leading to half of the menu being off the top of the screen and unusable.
This does not happen to avatars that do not use custom sitting animations. I've not been able to test behavior of this while in VR when using a custom sitting animation.
Replication:
Change into avatar with custom sitting animation
Go prone, to enter the animation
open the quick menu
select Avatar or worlds