ThreeJS Background Issue

A scene setup that shows an issue with backgrounds.

HTML

<script src="https://vanruesc.github.io/postprocessing/public/three.js"></script>
<script src="https://vanruesc.github.io/postprocessing/public/orbit-controls.js"></script>

JavaScript

(function() { "use strict";

	/**
	 * Loads assets.
	 *
	 * @method loadAssets
	 */

	(function loadAssets() {

		const loadingManager = new THREE.LoadingManager();
		const cubeTextureLoader = new THREE.CubeTextureLoader(loadingManager);

		const assets = {};

		loadingManager.onProgress = function(item, loaded, total) {

			if(loaded === total) { setupScene(assets); }

		};

		const path = "https://vanruesc.github.io/postprocessing/public/textures/skies/space3/";
		const format = ".jpg";
		const urls = [
			path + "px" + format, path + "nx" + format,
			path + "py" + format, path + "ny" + format,
			path + "pz" + format, path + "nz" + format
		];

		cubeTextureLoader.setCrossOrigin("");
		cubeTextureLoader.load(urls, function(textureCube) {

			assets.sky = textureCube;

		});

	}());

	/**
	 * Creates the scene and initiates the render loop.
	 *
	 * @method setupScene
	 * @param {Object} assets - Preloaded assets.
	 */

	function setupScene(assets) {

		const viewport = document.body;
		const width = window.innerWidth;
		const height = 150;
		const aspect = width / height;

		// Renderer and Scene.

		const renderer = new THREE.WebGLRenderer({
			antialias: true,
			logarithmicDepthBuffer: true // false
		});

		renderer.setClearColor(0x000000);
		renderer.setSize(width, height);
		viewport.appendChild(renderer.domElement);

		const scene = new THREE.Scene();
		scene.fog = new THREE.FogExp2(0x2d200f, 0.0025);

		// Sky.

		scene.background = assets.sky;

		// Camera.

		const camera = new THREE.PerspectiveCamera(50, aspect, 0.1, 2000);
		const controls = new THREE.OrbitControls(camera, renderer.domElement);
		controls.target.set(0, 0, 0);
		controls.enablePan = false;
		controls.minDistance = 2.5;
		camera.position.set(1, 1, 3);
		camera.lookAt(controls.target);

		scene.add(camera);

		// Lights.

		const ambientLight = new THREE.AmbientLight(0x888888);
		const directionalLight = new THREE.DirectionalLight(0xffbbaa);

		directionalLight.position.set(1, 1,...