Can I get world-space z-near-plane vertices from projection matrix + z-near distance?

Thanks @DMGregory for the pointer in the comments.

I'm constructing a fullscreen quad at (x, y, -1, 1) in NDC. Notice z = -1 is the near plane in NDC (left-handed unlike normal coordinate systems in OpenGL-land).

In the vertex shader:

attribute vec2 aVertexPosition;

uniform mat4 uInvViewMatrix;
uniform mat4 uInvProjMatrix;

varying vec3 vZNearPos;
varying vec3 vRayOrig;

void main() {
  // Vertices in the near plane in NDC
  gl_Position = vec4(aVertexPosition.xy, -1.0, 1.0);

  // Vertices in the near plane in world-space
  vec4 vZNearPosW = uInvViewMatrix * uInvProjMatrix * gl_Position;
  vZNearPos = vZNearPosW.xyz / vZNearPosW.w;

  // Ray origin in world-space
  // This could be a uniform but I was lazy :P
  vec4 rayOrigW = uInvViewMatrix * vec4(0, 0, 0, 1);
  vRayOrig = rayOrigW.xyz / rayOrigW.w;
}

The uniforms in JS:

• uInvViewMatrix is view.transform.matrix, NOT view.transform.inverse.matrix which you'd normally use (see https://developer.mozilla.org/en-US/docs/Web/API/XRView/transform). I.e. this is the inverse of the view matrix.
• uInvProjMatrix is the inverse of WebXR's view.projectionMatrix.

Then in the fragment shader:

varying vec3 vZNearPos;
varying vec3 vRayOrig;

void main() {
  // I'm not sure if this can be safely interpolated per-vertex
  // so I kept in the fragment shader just in case
  vec3 rayDir = normalize(vZNearPos - vRayOrig);

  // Paint your VR here!
}