src/vs/workbench/browser/composite.ts/Composite/registerFocusTrackEvents
class Composite:	private registerFocusTrackEvents(): { onDidFocus: Emitter<void>, onDidBlur: Emitter<void> } {
		const container = assertIsDefined(this.getContainer());
		const focusTracker = this._register(trackFocus(container));

		const onDidFocus = this._onDidFocus = this._register(new Emitter<void>());
		this._register(focusTracker.onDidFocus(() => {
			this._hasFocus = true;
			onDidFocus.fire();
		}));

		const onDidBlur = this._onDidBlur = this._register(new Emitter<void>());
		this._register(focusTracker.onDidBlur(() => {
			this._hasFocus = false;
			onDidBlur.fire();
		}));

		return { onDidFocus, onDidBlur };
	}