Scroll to selected Treeitem doesn't work for the first output

<window id="win" title="tree checkmark demo" border="normal" height="600px">
	<zscript>
Treeitem posItem1;
Treeitem posItem2;
void create() {
  for(int i=0; i != 45; i++) {
	Treeitem item = new Treeitem();
	Treerow row = new Treerow();
	Treecell cell = new Treecell();
	cell.label = "Item " + i;
	row.appendChild(cell);
	item.appendChild(row);
	
	if (i==0) {
	   posItem1 = item;
	}
	if (i==44) {
	   posItem2 = item;
	   // item.selected = true;  // doesn't help
	}
	child.appendChild(item);
  }
  tree.selectedItem= posItem2;
}
</zscript>
	<button label="Pos1" onClick="tree.selectedItem = posItem1;"/>
	<button label="Pos2" onClick="tree.selectedItem = posItem2;" />
	<separator bar="true" />
	<tree id="tree" width="90%"  vflex="true"
		checkmark="false"  rows="20" onCreate="create()">
		<treechildren id="child">
		</treechildren>
	</tree>
</window>