This is the latest released version (1.6.0) of the mod Fullscreen Windowed (Borderless) by hancin for MC 1.12-1.12.2, plus a fix for window resizing by me, compiled by Brycey92. This fork will be obsolete if this PR is merged.
This fixes the bug where the window is not resizable after exiting fullscreen. This is a bug in LWJGL, and is fixed by Fullscreen Fix and in current versions of Forge. However, the existing fixes are not applied to Fullscreen Windowed (Borderless), so this fork fixes it in that mod.
See the original Github post by Brycey92 here.