docs: function (position, size, base) {
              var viewportSize = this._getViewportSize();
              var documentScroll = this._getDocumentScroll(base);
  
              var minTopValue = documentScroll.scrollTop;
              var maxTopValue = Math.max(0, documentScroll.scrollTop + viewportSize.height - size.height);
              var top = aria.utils.Math.normalize(position.top, minTopValue, maxTopValue);
  
              var minLeftValue = documentScroll.scrollLeft;
              var maxLeftValue = Math.max(0, documentScroll.scrollLeft + viewportSize.width - size.width);
              var left = aria.utils.Math.normalize(position.left, minLeftValue, maxLeftValue);
  
              return {
                  'top' : top,
                  'left' : left
              };
          }