Note: This is copied from https://github.com/Brycey92/Fullscreen-Windowed-Minecraft/releases/tag/1.6.0-1, posted here to consolidate development of the mod in one place
This is the latest released version (1.6.0) of the mod by hancin for MC 1.12-1.12.2, plus a fix for window resizing by comp500, compiled by Brycey92.
Usually, when exiting fullscreen in newer versions of Minecraft, you can no longer maximize or resize the window due to a bug in LWJGL. Forge fixed the bug in a newer version, but many players need or prefer to remain on recommended versions of Forge.
Hancin, comp500, and Brycey92 claim no responsibility for supporting this unofficial release, nor liability for damages or issues it may cause. That being said, I don't really expect it to cause any issues.