InstancedMesh's setMatrixAt fails when another mesh in the scene has the same material

Description of the problem

I’m having a problem where the InstancedMesh method setMatrixAt seems to fail (the matrix positions doesn’t change visually) whenever I add another object with the same material object (not a clone) in the scene. No log/warning/errors are emitted.

The following briefly illustrates:

    var geometry = new THREE.BoxBufferGeometry();
    var material = new THREE.MeshStandardMaterial( { roughness: 0, envMap: texture });
    var mesh1 = new THREE.Mesh( geometry, material );
    mesh1.position.set(0, 1, 0);
    scene.add( mesh1 ); // removing this line makes the instanced mesh work as intended
    mesh = new THREE.InstancedMesh( geometry, material, 9 ); // same material usage
    scene.add( mesh ); // This works and I can see both meshes in the scene.
    
    var dummy = new THREE.Object3D();
    dummy.position.set( 1, 5, 1 );
    dummy.updateMatrix();
    mesh.setMatrixAt( 0, dummy.matrix ); // Nope, doesn't work, it just stays where it's at.
    mesh.setMatrixAt( 1, dummy.matrix ); // Rotation seems to be applied, but position doesn't.
    // It looks like every instance is at the same place, but I don't know.

Note that if I use material.clone() at the instanced mesh declaration, the problem also goes away / is fixed, but now I have two materials, and I intend to have one for performance reasons.

A live example is available here:

• jsfiddle (The problem is that you should see multiple boxes, but only 1 or 2 are visible, remove line 41 to fix)

Three.js version

• Dev
• r109
• r108

Browser

• All of them
• Chrome
• Firefox
• Internet Explorer

OS

• All of them
• Windows
• macOS
• Linux
• Android
• iOS

Hardware Requirements (graphics card, VR Device, …)

(Not applicable, as far as I know)

[Mugen87]

@mracette Good point. Let me update the docs.

[mracette]

I just got bit by this as well. A note in the docs while the fix is underway would be great. https://threejs.org/docs/#api/en/objects/InstancedMesh