How to make sure GoJS TreeView node does not change position by scroll down?

@sharpost wrote:

The tree view height is less than the div height. So there will be empty space at bottom. When user scrolls down, the tree view shifts to bottom. How can i avoid that?
It works fine when tree view height is same or more (having scroll bar)

gojs tree scroll.png647x842 14.1 KB

GoJS Tree View

<script id="code">
  function init() {
    if (window.goSamples) goSamples();  // init for these samples -- you don't need to call this
    var $ = go.GraphObject.make;  // for conciseness in defining templates

    myDiagram =
      $(go.Diagram, "myDiagramDiv",
        {
            allowMove: true,
            allowCopy: true,
            allowDelete: false,
            allowDragOut: true,
            allowDrop: true,
            allowZoom: false,
            "animationManager.isEnabled": false,
            allowHorizontalScroll: false,

          layout:
            $(go.TreeLayout,
              {
                alignment: go.TreeLayout.AlignmentStart,
                angle: 0,
                compaction: go.TreeLayout.CompactionNone,
                layerSpacing: 16,
                layerSpacingParentOverlap: 1,
                nodeIndent: 2,
                nodeIndentPastParent: 0.88,
                nodeSpacing: 0,
                setsPortSpot: false,
                setsChildPortSpot: false
              })
      });

    myDiagram.nodeTemplate =
      $(go.Node,
        { // no Adornment: instead change panel background color by binding to Node.isSelected
          selectionAdorned: false,
          // a custom function to allow expanding/collapsing on double-click
          // this uses similar logic to a TreeExpanderButton
          doubleClick: function(e, node) {
            var cmd = myDiagram.commandHandler;
            if (node.isTreeExpanded) {
              if (!cmd.canCollapseTree(node)) return;
            } else {
              if (!cmd.canExpandTree(node)) return;
            }
            e.handled = true;
            if (node.isTreeExpanded) {
              cmd.collapseTree(node);
            } else {
              cmd.expandTree(node);
            }
          }
        },
        $("TreeExpanderButton",
          {
            width: 14,
            "ButtonBorder.fill": "whitesmoke",
            "ButtonBorder.stroke": null,
            "_buttonFillOver": "rgba(0,128,255,0.25)",
            "_buttonStrokeOver": null
          }),
        $(go.Panel, "Horizontal",
          { position: new go.Point(16, 0) },
          new go.Binding("background", "isSelected", function (s) { return (s ? "lightblue" : "white"); }).ofObject(),
          $(go.Picture,
            {
              width: 18, height: 18,
              margin: new go.Margin(0, 4, 0, 0),
              imageStretch: go.GraphObject.Uniform
            },
            // bind the picture source on two properties of the Node
            // to display open folder, closed folder, or document
            new go.Binding("source", "isTreeExpanded", imageConverter).ofObject(),
            new go.Binding("source", "isTreeLeaf", imageConverter).ofObject()),
          $(go.TextBlock,
            { font: '9pt Verdana, sans-serif' },
            new go.Binding("text", "key", function(s) { return "item " + s; }))
        )  // end Horizontal Panel
      );  // end Node

    // without lines
    myDiagram.linkTemplate = $(go.Link);

    var nodeDataArray = [{ key: 0 }];
    var max = 20;
    var count = 0;
    while (count < max) {
      count = makeTree(3, count, max, nodeDataArray, nodeDataArray[0]);
    }
    myDiagram.model = new go.TreeModel(nodeDataArray);
  }

  function makeTree(level, count, max, nodeDataArray, parentdata) {
    var numchildren = Math.floor(Math.random() * 10);
    for (var i = 0; i < numchildren; i++) {
      if (count >= max) return count;
      count++;
      var childdata = { key: count, parent: parentdata.key };
      nodeDataArray.push(childdata);
      if (level > 0 && Math.random() > 0.5) {
        count = makeTree(level - 1, count, max, nodeDataArray, childdata);
      }
    }
    return count;
  }

  // takes a property change on either isTreeLeaf or isTreeExpanded and selects the correct image to use
  function imageConverter(prop, picture) {
    var node = picture.part;
    if (node.isTreeLeaf) {
      return "images/document.png";
    } else {
      if (node.isTreeExpanded) {
        return "images/openFolder.png";
      } else {
        return "images/closedFolder.png";
      }
    }
  }
</script>
</head>
<body onload="init()">
<div id="sample">
  <div id="myDiagramDiv" style="border: 1px solid black; width: 500px; height: 650px"></div>
</div>
</body>
</html>

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