src/vs/platform/windows/electron-main/windowsMainService.ts/WindowsMainService/doGetLastActiveWindow
class WindowsMainService:	private doGetLastActiveWindow(windows: ICodeWindow[]): ICodeWindow | undefined {
		const lastFocusedDate = Math.max.apply(Math, windows.map(window => window.lastFocusTime));

		return windows.find(window => window.lastFocusTime === lastFocusedDate);
	}