A library for Minecraft Forge 1.12.2 that allows large rectangular textures (e.g. 7×4, 20×12) to be used safely in the vanilla atlas — fixing crashes, cropping/corruption, seams, and incorrect behavior with CTM patterns.
No code integration is needed: adding a .png.mcmeta file next to the texture is enough, and the fixes are applied automatically only where they're needed.
Minecraft 1.12.2; Forge 14.23.x; ConnectedTexturesMod (CTM) for pattern textures.
The complete explanation (how it works, .mcmeta examples, limitations) is on the mod page:
Special thanks to the Chisel Team for ConnectedTexturesMod (CTM), which inspired part of the solution and made pattern textures possible in the first place.