How to add panel node in treeview control ?

TreeView doesn't have a Scroll event. It isn't reliable anyway since nodes can be expanded and collapsed, changing the position and visibility of the node. Alternative is to use timer as below:

private void timer1_Tick(object sender, EventArgs e) {
    var node = treeView1.SelectedNode;
    if (node == null || !node.IsVisible) panel1.Visible = false;
    else {
        panel1.Visible = true;
        var nodepos = treeView1.PointToScreen(node.Bounds.Location);
        var panelpos = panel1.Parent.PointToClient(nodepos);
        panel1.Top = panelpos.Y;
    }
}

It works well to clarify your requirement.