Workaround would be to apply the first-column indicator CSS class name in the markup. Then this class name will be inherited by the next column if the first is Hidden.

With the following markup, the rendered results, the "Parent Category ID" column header will have the "first-visible-column" CSS class name set.
<telerik:TreeListBoundColumn DataField="ID" UniqueName="ID" HeaderText="Category ID" Visible="false">
    <HeaderStyle CssClass="first-visible-column" />
</telerik:TreeListBoundColumn>

<telerik:TreeListBoundColumn DataField="ParentID" UniqueName="ParentID" HeaderText="Parent Category ID">
</telerik:TreeListBoundColumn>
OR

If applying in the Code Behind, try using the PreRenderComplete event of the Page.

protected void Page_PreRenderComplete(object sender, EventArgs e)
{
    string firstVisibleColName = RadTreeList1.RenderColumns.Where(x => x.Visible && x.Display).First().UniqueName;

    TreeListHeaderItem headerItem = RadTreeList1.GetItems(TreeListItemType.HeaderItem)[0] as TreeListHeaderItem;
    headerItem[firstVisibleColName].CssClass += " first-visible-column";
}

The agregate sum value shown in the bottom of the TreeList columns displays bigger sum than the actual one (it seems that the child items values are also taken into consideration).

Steps to reproduce:
1. Open http://demos.telerik.com/aspnet-ajax/treelist/examples/columns/aggregates/defaultcs.aspx?show-source=true

2. Check the sum shown in the Population column

Actual: It shows 10,702,470,081.00

Expected: It should be 6,464,732,588.00

This error occurs using version 2013.2.717.45 of Telerik.Web.UI.dll

We are using a RadTreeList to display data with a three-part key.  The control was set up with AllowLoadOnDemand="true", DataKeyNames="Key1,Key2,Key3", and ParentDataKeyNames="ParentKey1,ParentKey2,ParentKey3", and both the NeedDataSource and ChildItemsDataBind events were handled properly on the server side.  Clicking the expand/collapse buttons resulted in erratic behavior -- sometimes, all nodes on a given level would be expanded or collapsed at once, sometimes, the entire tree would collapse and not be able to be expanded again, and occasionally the desired node would expand or collapse as desired.

I tracked the problem (using JustDecompile) to a bug in TreeListLoadOnDemandContext.ItemNeedsToBeExpanded.  The same bug appears to be in ItemNeedsToBeSelected, RemoveExpandedDataKeyValue, and RemoveSelectedDataKeyValue as well.  In all four cases, the code loops through the data keys comparing one to the next -- but overwrites its flag on each iteration.  In other words, it doesn't check if ALL keys are equal, only if the last one compared is equal.


I added "Key" and "ParentKey" fields to the data object that combined all three fields into a single struct, so that the control now has DataKeyNames="Key" and ParentDataKeyNames="Parent", and it now works properly.