code: private void updateVerticalMargin() {
  
          int floatHeight = m_floatBox.getOffsetHeight();
          int primaryHeight = m_primary.getOffsetHeight();
          int verticalOffset = (primaryHeight - floatHeight) / 2;
          m_floatBox.getElement().getStyle().setMarginTop(verticalOffset, Unit.PX);
      }
docs: Updates the vertical margin of the float box such that its vertical middle point coincides
  with the vertical middle point of the primary panel.<p>