Found a solution. What I had was mostly correct, but I needed to flush the ZBuffer, then re-draw my portal triangles to the depth buffer after drawing the contents of the portal.

void RENDER_StartPolyStencil(vertexStruct* v, int n)
{
	glEnable(GL_STENCIL_TEST);
	glDepthMask(GL_TRUE);

	glStencilMask…

I found a pretty great tutorial on how to do clipping in OpenGL 4: https://prideout.net/clip-planes​

Do you have a favourite website or code to do clipping in OpenGL 4?

For the post above include some relative translation to the shader so the math involved wont have to operate on vey large numbers….