This mod replaces the standard Minecraft fullscreen mode with a custom borderless windowed fullscreen. This is especially useful in a multi-monitor scenario to easily switch to a 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. Furthermore, other windows can be placed on top of the Minecraft window.
This mod works great with Forge and OptiFine.
Go to the CurseForge project page for further details.
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).