src/vs/workbench/contrib/notebook/browser/view/renderers/webviewPreloads.ts/OutputFocusTracker/constructor
class OutputFocusTracker:		constructor(element: HTMLElement | Window, outputId: string) {
			this._element = element;
			this._outputId = outputId;
			this._hasFocus = isAncestor(document.activeElement, <HTMLElement>element);
			this._loosingFocus = false;

			element.addEventListener('focus', this._onFocus.bind(this), true);
			element.addEventListener('blur', this._onBlur.bind(this), true);
		}