This HUD prevents landscape mirroring by letterboxing the 3D world view window. It automatically and dynamically resizes the view window whenever the resolution of the game window increases or decreases, whenever sniper mode is toggled on or off, whenever extravision is picked up or expires... In other words, whenever the FOV changes, no matter the cause (teleportation effect excluded).

Here's a little video showcasing how it works (16:9 screen, initially 90° FOV, then sniper mode is toggled on and then off).

It is based on Basic HUD by treellama, and has the same compatibilities as that HUD with regards to scenarios (for example, it is compatible with Marathon 2: Durandal, Marathon Infinity and Rubicon X, but not with Marathon).