Letterboxed
This is a fun little mod I made because I started playing Minecraft (Java) on an 16:10 Monitor while now I use a 16:9 one.
You can set any aspect ratio you want and the mod will keep it preserved adding black bars on the sides or top and bottom.
You can configure the mod via the /letterboxed command. Either set a ratio directly via /letterboxed <ratio> or indirectly via /letterboxed <width> <height> (the ratio will be width / height). /letterboxed reset is basically just /letterboxed set 0.
Setting the ratio to 0 will effectively disable the Mod.
I'm probably going to add some kind of GUI to configure the Mod (idk yet).
As the mod doesn't just draw black bars to the screen, but also move the GUI around, there are probably many Mods that won't render their UI correctly with this mod installed.
The mod will also only affect the GUI not the actual 3d world. So the black bars can overlap important stuff like your hands if you use to high or low ratios.
There are still many issues even in vanilla (screens sometimes being cut off, mouse behaving weird with scrollbars, ...) so I'll try to fix them in future releases.
