src/vs/base/browser/dom.ts/FocusTracker/hasFocusWithin
class FocusTracker:	private static hasFocusWithin(element: HTMLElement): boolean {
		const shadowRoot = getShadowRoot(element);
		const activeElement = (shadowRoot ? shadowRoot.activeElement : document.activeElement);
		return isAncestor(activeElement, element);
	}