Screen-aligned quad in clipspace is drawing atop nearer elements - scenekit

Question

I have a textured quad that needs to be drawn in screenspace, sized/aligned to the viewport, in a SceneKit scene at the far plane - think of it like a textured quad to act as a background fill.
I've manually created a 4 vertex quad in clip space, and written a vertex/fragment shader to draw it. The vertices are set to be at z=1, which should place them at the far plane.
However, while the quad is correctly rendered as expected in screen space (e.g., it is correctly aligned) it is drawn atop all scene contents - with z=1 for the clip space vertices it should be behind everything.
I've been able to work around this by setting a low renderingOrder and disabling depth writes, but that's not a fix, that's a hack.
// build the plane vertices in clip space
let vertices: [SCNVector3] = [
SCNVector3(-1, -1, +1),
SCNVector3(+1, -1, +1),
SCNVector3(+1, +1, +1),
SCNVector3(-1, +1, +1),
]
let texCoords: [CGPoint] = [
CGPoint(x: 0, y: 1),
CGPoint(x: 1, y: 1),
CGPoint(x: 1, y: 0),
CGPoint(x: 0, y: 0),
]
let indices: [UInt16] = [
0,1,2,
0,2,3,
]
let vertexSource = SCNGeometrySource(vertices: vertices)
let texCoordSource = SCNGeometrySource(textureCoordinates: texCoords)
let elementSource = SCNGeometryElement(indices: indices, primitiveType: .triangles)
let planeGeometry = SCNGeometry(sources: [vertexSource, texCoordSource], elements: [elementSource])
let program = SCNProgram()
program.library = ShaderUtilities.metalLibrary
program.vertexFunctionName = "plane_vertex"
program.fragmentFunctionName = "plane_fragment"
if let material = planeGeometry.firstMaterial {
material.program = program
material.setValue(SCNMaterialProperty(contents: backdropImage as Any), forKey: "backgroundTexture")
}
let planeNode = SCNNode(geometry: planeGeometry)
meshCameraNode.addChildNode(planeNode)
The corresponding metal shaders:
struct PlaneVertexIn {
float3 position [[attribute(SCNVertexSemanticPosition)]];
float2 uv [[attribute(SCNVertexSemanticTexcoord0)]];
};
struct PlaneVertexOut {
float4 position [[position]];
float2 uv;
};
vertex PlaneVertexOut plane_vertex(PlaneVertexIn in [[ stage_in ]],
constant SCNSceneBuffer& scn_frame [[buffer(0)]],
constant NodeBuffer& scn_node [[buffer(1)]]) {
PlaneVertexOut out;
// the vertices are already in clip space to form a screen-aligned quad, so no need to apply a transform.
out.position = float4(in.position.x, in.position.y, in.position.z, 1.0);
out.uv = in.uv;
return out;
}
fragment float4 plane_fragment(PlaneVertexOut out [[ stage_in ]],
constant SCNSceneBuffer& scn_frame [[buffer(0)]],
texture2d<float, access::sample> backgroundTexture [[texture(0)]]) {
constexpr sampler textureSampler(coord::normalized, filter::linear, address::repeat);
return backgroundTexture.sample(textureSampler, out.uv);
}