debug_focus_tracking = (document, container_el, descendant_el, is_root) => {
		if (!$Window.DEBUG_FOCUS) {
			return;
		}
		let svg = debug_svg_by_container.get(container_el);
		if (!svg) {
			svg = document.createElementNS("http://www.w3.org/2000/svg", "svg");
			svg.style.position = "fixed";
			svg.style.top = "0";
			svg.style.left = "0";
			svg.style.width = "100%";
			svg.style.height = "100%";
			svg.style.pointerEvents = "none";
			svg.style.zIndex = "100000000";
			svg.style.direction = "ltr"; // position labels correctly
			debug_svg_by_container.set(container_el, svg);
			debug_svgs_in_window.push(svg);
			document.body.appendChild(svg);
		}
		svg._container_el = container_el;
		svg._descendant_el = descendant_el;
		svg._is_root = is_root;
		animate_debug_focus_tracking();
	}
@param {Document} document 
@param {Element} container_el 
@param {Element} descendant_el 
@param {boolean} is_root