By the UI they mean the controls inside of the page, not the browser's UI - your back, refresh and other browser controls still work. The same thing happens if

By the UI they mean the controls inside of the page, not the browser's UI - your back, refresh and other browser controls still work. The same thing happens if you have javascript that consumes CPU and doesn't return for a long period of time. While the javascript is running controls on the page don't work such as buttons and select lists, but when the javascript finishes the UI thread regains control and responds to input again.