Borderless Window 
This Forge mod replaces the standard Minecraft fullscreen mode with a custom borderless fullscreen. This is especially useful when using multiple monitors to switch to another program on a different monitor. The default behavior of the Minecraft fullscreen would minimize the game once it loses focus. With this mod, the window will not minimize when switching to another program. In addition, other windows can be placed on top of the Minecraft window.
This mod works with Forge and OptiFine.
Have a look at Borderless Mining by comp500 if you are searching for a similar mod for Fabric.
Installation
As usual, locate your Minecraft installation folder and place the downloaded JAR file in your "mods" folder. That's it!
The window should now automatically be borderless when switching to fullscreen (i.e., pressing F11 or activating the fullscreen from the Video Settings).
FAQ
Can I include this mod in my modpack?
Sure, you are free to include this mod in any modpack you want. Since this project is published under the MIT License, you can even change the source code and include a modified version if you want!
Does this mod work on Windows/Linux/Mac?
Windows and Linux using X11 or Wayland will work just fine. Borderless Window is disabled on macOS.
The game freezes on startup, and the window doesn't respond. What can I do?
When using many mods, the window might hang up and will not respond on startup. This is a Minecraft/Forge related issue, just wait until the main menu is loaded. Unfortunately, this is the default window behavior and is not related to Borderless Window.
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The game has serious performance issues with this mod. What can I do?
Using a borderless fullscreen might impact the game's performance. You'll always get less FPS with a borderless fullscreen instead of a normal video mode fullscreen.
There's an internal bug in the window library that Minecraft uses. It might enable VSync when the window is in borderless fullscreen, causing significant performance issues. At the moment, there is nothing I can do to fix this bug until a new version of the library is released.
