When using Better Portals and p455w0rdslib you get graphical issues:

Screenshots were taken on Minecraft version 1.12.2 with

• Forge 1.12.2-14.23.5.2838
• p455w0rdslib-1.12.2-2.2.156.jar (md5sum 56a57b60eff38f98cf0228b802df2322)
• betterportals-0.3.7.1.jar (md5sum d11bf3d057509a11b8738df604094d46)

Workaround

This issue can be worked around by setting EnableShaders to false in p455w0rdsLib.cfg.

I've opened a related issue in p455w0rds-Library's repository: p455w0rd/p455w0rds-Library#54

p455w0rdslib must in its vertex shader set gl_ClipVertex to the correct value (it doesn't currently set it at all).
GLSL spec (see section 7.1): https://www.khronos.org/registry/OpenGL/specs/gl/GLSLangSpec.1.20.pdf
This might also help: https://bugs.freedesktop.org/show_bug.cgi?id=64668#c12

thanks for the reply @Johni0702 ..my glsl knowledge is (i think obviously) limited. I wouldn't know how to calculate the clip values/what gl_ClipVertex should be set to. I've given a lot of thought to removing the shaders/coremod from pwlib and likely will since I don't have the time to continue learning about glsl.