Borderless Mining 
Borderless Mining changes the fullscreen option to use a borderless window that fills the screen. This allows you to have Minecraft open on one monitor, while using other programs on another monitor - as normally Minecraft minimises when you unfocus it.
This mod also adds the Borderless Windowed option to the Video Settings screen, so you can easily disable the mod ingame, and has a configuration menu accessible through Mod Menu.
This mod is a rewrite of a Forge 1.12 mod, Fullscreen Windowed (Borderless), however it used to be a fork of that mod (which the 1.12 version of this mod is) - see the bottom of the Curseforge page for the original description.
See Borderless Window by nekeras if you're looking for a similar mod for Forge 1.14+
On macOS this mod is disabled by default, as it is impossible to position the window above the menu bar, and macOS has better fullscreen support anyway.