What does this mod do?
It lets Minecraft stay in fullscreen mode when it becomes inactive (i.e., when you switch to another window). This was added to vanilla Minecraft 1.8 to fix bug MC-55045, which made it very difficult for some users to enter fullscreen at all.
How do I use this mod?
You need Minecraft Forge installed first. Once that's done, just drop fullscreenwhileinactive-version.jar in your Minecraft instance's mods/ directory.
Can I use this in my modpack?
Yes, even if you monetize it with adf.ly or something, and you don't need to ask me for my permission first.