code: static HandlerRegistration capturePageEvent (String name, EventHandler handler) {
      return addEventListener(Document.get(), name, handler, true);
    }
docs: Capture events that occur anywhere on the page. Event values will be relative to the page
  (not the rootElement) {@see #getRelativeX(NativeEvent, Element)} and
  {@see #getRelativeY(NativeEvent, Element)}.