code: public void setWidgetSnapClosedSize(Widget child, int snapClosedSize) {
          assertIsChild(child);
          Splitter splitter = getAssociatedSplitter(child);
          // The splitter is null for the center element.
          if (splitter != null) {
              splitter.setSnapClosedSize(snapClosedSize);
          }
      }
docs: Sets a size below which the slider will close completely. This can be used
  in conjunction with {@link #setWidgetMinSize} to provide a speed-bump
  effect where the slider will stick to a preferred minimum size before
  closing completely.
  
  <p>
  This method has no effect for the {@link DockLayoutPanel.Direction#CENTER}
  widget.
  </p>
  
  @param child the child whose slider should snap closed
  @param snapClosedSize the width below which the widget will close or
  -1 to disable.