code: protected void beginDragging(MouseDownEvent event) {
  
          m_dragging = true;
          m_windowWidth = Window.getClientWidth();
          m_clientLeft = Document.get().getBodyOffsetLeft();
          m_clientTop = Document.get().getBodyOffsetTop();
          DOM.setCapture(getElement());
          m_dragStartX = event.getX();
          m_dragStartY = event.getY();
          addStyleName(I_CmsLayoutBundle.INSTANCE.dialogCss().dragging());
      }
docs: Called on mouse down in the caption area, begins the dragging loop by
  turning on event capture.
  
  @see DOM#setCapture
  @see #continueDragging
  @param event the mouse down event that triggered dragging