GLTFLoader: Fix EXT_mesh_gpu_instancing custom instance attributes shared across nodes (#33927)

Co-authored-by: Michael Herzog <michael.herzog@human-interactive.org>

examples/jsm/loaders/GLTFLoader.js

@@ -1786,6 +1786,9 @@ class GLTFMeshGpuInstancing {
 				}
 
 				// Add instance attributes to the geometry, excluding TRS.
+
+				let instanceGeometry = null;
+
 				for ( const attributeName in attributes ) {
 
 					if ( attributeName === '_COLOR_0' ) {
@@ -1797,7 +1800,36 @@ class GLTFMeshGpuInstancing {
 						 attributeName !== 'ROTATION' &&
 						 attributeName !== 'SCALE' ) {
 
-						mesh.geometry.setAttribute( attributeName, attributes[ attributeName ] );
+						if ( instanceGeometry === null ) {
+
+							// do a shallow clone of the goemetry so per-instance data are not shared
+
+							const source = instancedMesh.geometry;
+							instanceGeometry = new BufferGeometry();
+							instanceGeometry.name = source.name;
+
+							for ( const name in source.attributes ) instanceGeometry.setAttribute( name, source.attributes[ name ] );
+							for ( const name in source.morphAttributes ) instanceGeometry.morphAttributes[ name ] = source.morphAttributes[ name ];
+							if ( source.index !== null ) instanceGeometry.setIndex( source.index );
+
+							instanceGeometry.morphTargetsRelative = source.morphTargetsRelative;
+
+							for ( const group of source.groups ) instanceGeometry.addGroup( group.start, group.count, group.materialIndex );
+
+							if ( source.boundingBox !== null ) instanceGeometry.boundingBox = source.boundingBox.clone();
+							if ( source.boundingSphere !== null ) instanceGeometry.boundingSphere = source.boundingSphere.clone();
+
+							instanceGeometry.drawRange.start = source.drawRange.start;
+							instanceGeometry.drawRange.count = source.drawRange.count;
+
+							instanceGeometry.userData = Object.assign( {}, source.userData );
+
+							instancedMesh.geometry = instanceGeometry;
+
+						}
+
+						const attr = attributes[ attributeName ];
+						instanceGeometry.setAttribute( attributeName, new InstancedBufferAttribute( attr.array, attr.itemSize, attr.normalized ) );
 
 					}