To reproduce:

DataTable dt = new DataTable();
 
public Form1()
{
    InitializeComponent();

    dt.Columns.Add("Id", typeof(int));
    dt.Columns.Add("ParentID", typeof(int));
    dt.Columns.Add("NodeName", typeof(string));
    dt.Rows.Add(1, 0, "Category1");
    dt.Rows.Add(2, 1, "Category2");
    dt.Rows.Add(3, 0, "Category3");
    dt.Rows.Add(4, 3, "Category4");
    dt.Rows.Add(5, 3, "Category5");
    dt.Rows.Add(6, 3, "Category6");
    dt.Rows.Add(7, 6, "Category7");
    dt.Rows.Add(8, 6, "Category8"); 

    this.radTreeView1.DataSource = dt; 

    this.radTreeView1.DisplayMember = "NodeName";
    this.radTreeView1.ChildMember = "ID";
    this.radTreeView1.ParentMember = "ParentID";

    this.radTreeView1.ExpandAll();

    this.radTreeView1.AllowAdd = true;
    this.radTreeView1.AllowRemove = true;
    this.radTreeView1.AllowDefaultContextMenu = true;

    this.radLabel1.Text = "Number of rows: " + dt.Rows.Count;
    
    this.radTreeView1.NodeRemoved += radTreeView1_NodeRemoved;
}

private void radTreeView1_NodeRemoved(object sender, Telerik.WinControls.UI.RadTreeViewEventArgs e)
{
    this.radLabel1.Text = "Number of rows: " + dt.Rows.Count;
}

Workaround: perform the delete operation programmatically by modifying the default context menu:

 this.radTreeView1.ContextMenuOpening += radTreeView1_ContextMenuOpening;

private void radTreeView1_ContextMenuOpening(object sender, TreeViewContextMenuOpeningEventArgs e)
{
    foreach (RadMenuItem item in e.Menu.Items)
    {
        if (item.Text == "&Delete")
        {
            item.Visibility = Telerik.WinControls.ElementVisibility.Collapsed;
        }
        else
        {
            item.Visibility = Telerik.WinControls.ElementVisibility.Visible;
        }
    }

    RadMenuItem removeItem = new RadMenuItem("Remove");
    e.Menu.Items.Add(removeItem);
    removeItem.Click += item_Click;
}

private void item_Click(object sender, EventArgs e)
{
    DeleteSubNodes(this.radTreeView1.SelectedNode);
    this.radLabel1.Text = "Number of rows: " + dt.Rows.Count;
}

private void DeleteSubNodes(Telerik.WinControls.UI.RadTreeNode node)
{
    if (node.Nodes.Count == 0)
    {
        dt.Rows.Remove(((DataRowView)node.DataBoundItem).Row);
    }
    else
    {
        foreach (RadTreeNode n in node.Nodes)
        {
            DeleteSubNodes(n);
        }
        dt.Rows.Remove(((DataRowView)node.DataBoundItem).Row);
    }
}

To reproduce:
- Open the Property builder, add some nodes and disable them.
- There is no way to enable them at design time. 

Workaround. 
Disable the nodes at runtime.

Steps to reproduce: 
1. Add RadTreeView with few items 
2. Apply custom filtering 
3. In order to remove the custom filtering and return the default filtering, set the FilterPredicate to null. 
As a result, it is thrown an exception. 

Workaround: 
1. Save the default filter predicate before applying the custom filtering 
2. Clear filter descriptors and set the FilterPredicate property to stored default filter predicate

In the attachments can be found a sample application demonstrating the issue and the workaround 

To reproduce: 

Add a RadTreeView and a RadBreadCrumb and two RadButtons on the form and populate the tree view with data. Use the following code snippet:

private void radButton1_Click(object sender, EventArgs e)
{
    this.radBreadCrumb1.DefaultTreeView = null;
}

Random rand = new Random();

private void radButton2_Click(object sender, EventArgs e)
{
    this.radTreeView1.DataSource = null;
    DataTable dt = new DataTable();
    dt.Columns.Add("Id");
    dt.Columns.Add("Name");
    for (int i = 0; i < 10; i++)
    {
        dt.Rows.Add(rand.Next(0, 20), "Node" + i);
    }
    this.radTreeView1.DataSource = dt;
    this.radTreeView1.DisplayMember = "Name";
    this.radTreeView1.ValueMember = "Id";

    this.radBreadCrumb1.DefaultTreeView = this.radTreeView1;
}

Select some node in RadTreeView, the RadBreadCrum is updated respectively. Then click the first button to detach RadTreeView from RadBreadCrumb. Click the second button to populate RadTreeView with new data and associated the RadBreadCrumb with RadTreeView again. As a result you will notice that the last selected node will be loaded.

Workaround: clear the selection in RadTreeView before associating the RadBreadCrumb with RadTreeView:

this.radTreeView1.ClearSelection();
this.radBreadCrumb1.DefaultTreeView = this.radTreeView1;

To reproduce:

this.radTreeView1.TriStateMode = true;

RadTreeNode sitesNode = radTreeView1.Nodes.Add("Sites", "Sites", "");
sitesNode.Expanded = true;

RadTreeNode siteNode = new RadTreeNode("A site");
sitesNode.Nodes.Add(siteNode);

RadTreeNode eNode = new RadTreeNode("Event Types");
siteNode.Nodes.Add(eNode);
siteNode.Expanded = true;
RadTreeNode dummy = new RadTreeNode("dummy");
eNode.Nodes.Add(dummy);
dummy.CheckState = ToggleState.On;

RadTreeNode dummy2 = new RadTreeNode("dummy2");
eNode.Nodes.Add(dummy2);
dummy2.CheckState = ToggleState.Off;

Please refer to the attached screenshot. 

Workaround: change the CheckState property after all nodes are added to RadTreeView.

this.radTreeView1.TriStateMode = true;

RadTreeNode sitesNode = radTreeView1.Nodes.Add("Sites", "Sites", "");
sitesNode.Expanded = true;

RadTreeNode siteNode = new RadTreeNode("A site");
sitesNode.Nodes.Add(siteNode);

RadTreeNode eNode = new RadTreeNode("Event Types");
siteNode.Nodes.Add(eNode);
siteNode.Expanded = true;
RadTreeNode dummy = new RadTreeNode("dummy");
eNode.Nodes.Add(dummy);


RadTreeNode dummy2 = new RadTreeNode("dummy2");
eNode.Nodes.Add(dummy2);

dummy2.CheckState = ToggleState.Off; 
dummy.CheckState = ToggleState.On;

To reproduce: please refer to the attached gif file.

Workaround: insteda of hiding the expander by the ShowExpandCollapse property use the NodeFormatting event as follows:

private void radTreeView1_NodeFormatting(object sender, TreeNodeFormattingEventArgs e)
        {
            e.NodeElement.ExpanderElement.Visibility = ElementVisibility.Collapsed;
        }

Please refer to the attached gif file and sample project. The screen tip position is not the same each time. Hence, the issue may not be reproducible every time.

Workaround: use the TooltiptextNeeded event.

To reproduce:
private void RadTreeView1_NodeCheckedChanging(object sender, Telerik.WinControls.UI.RadTreeViewCancelEventArgs e)
{
    Console.WriteLine(e.Action);
}

The action is always unknown.

Pressing arrow down or up when renaming item on a list with scrollbar throws null pointer exception. Example solution in attachement. 

Steps to reproduce with attached solution: 
Scroll down to last element (A99) 
Press F2 (rename) and rename to "0"
Press arrow up or down on a keyboard (without pressing enter after rename). -> NullReferenceException is thrown.

How to reproduce: check the attached project

Workaround: create a custom TreeViewDragDropService
class CustomTreeViewElement : RadTreeViewElement
{
    protected override Type ThemeEffectiveType
    {
        get
        {
            return typeof(RadTreeViewElement);
        }
    }

    protected override TreeViewDragDropService CreateDragDropService()
    {
        return new CustomDragDropService(this);
    }
}

class CustomTreeView : RadTreeView
{
    protected override RadTreeViewElement CreateTreeViewElement()
    {
        return new CustomTreeViewElement();
    }
 
    public override string ThemeClassName
    {
        get
        {
            return typeof(RadTreeView).FullName;
        }
    }
}

class CustomDragDropService : TreeViewDragDropService
{
    public CustomDragDropService(RadTreeViewElement owner) 
        : base(owner)
    { }

    protected override bool CancelPreviewDragDrop(RadDropEventArgs e)
    {
        return false;
    }
}

For the MS TreeView the HideSelection property gets or sets a value indicating whether the selected tree node remains highlighted even when the tree view has lost the focus. However, it doesn't work in RadTreeView.

When you start dragging a node you will notice that the drag hint is constantly flickering.

To reproduce: please refer to the attached gif files with the different versions of the Telerik UI for WinForms suite . You will notice that while you are dragging a node the default cursor is an arrow with a small rectangle. If you hold the Ctrl key pressed while dragging  the node, the default cursor is an arrow with a small rectangle and '+'. With the latest version, the '+' sign is missing when you hold the Ctrl key. 

To reproduce: please refer to the attached project.

Workaround: if you access the RadTreeNode.TreeViewElement property, the TreeView property is loaded.

To reproduce: add some nodes to the tree view at design time

        public RadForm1()
        {
            InitializeComponent();

            this.radTreeView1.AllowDragDrop = true;
            this.radTreeView1.DragEnding += radTreeView1_DragEnding;
        }
        
        private void radTreeView1_DragEnding(object sender, Telerik.WinControls.UI.RadTreeViewDragCancelEventArgs e)
        {
            if (MessageBox.Show("Are you sure you want to move?", "Question", MessageBoxButtons.YesNo) == System.Windows.Forms.DialogResult.No)
            {
                e.Cancel = true;
            }
        }

Then try to drag and drop a node. You will notice that the application hangs

Workaround: use RadMessageBox

At the moment the filter predicate is called only for the root nodes. A possible workaround is to recursively apply the filter logic for the child nodes as well.
Workaround:
private void Button1_Click(object sender, EventArgs e)
{
    this.radTreeView1.TreeViewElement.FilterPredicate = this.FilterNode;
    this.radTreeView1.Filter = "Custom";
}

private bool FilterNode(RadTreeNode node)
{
    Console.WriteLine(node.Text);
    if (node.Text.Contains("Child: 3"))
    {
        return true;
    }

    Stack<RadTreeNode> children = new Stack<RadTreeNode>();
    if (node.Nodes.Count > 0)
    {
        children.Push(node);
        while (children.Count > 0)
        {
            RadTreeNode current = children.Pop();
            foreach (RadTreeNode child in current.Nodes)
            {
                if (child.Text.Contains("Child: 3"))
                {
                    return true;
                }

                children.Push(child);
            }
        }
    }

    return false;
}

Workaround:
private void radButton1_Click(object sender, EventArgs e)
{
    var allNodes = radTreeView1.TreeViewElement.GetNodes().ToList();

    int row = 0;
  
    Workbook workbook = new Workbook();
    Worksheet newWorksheet = workbook.Worksheets.Add();

    foreach (var item in allNodes)
    {

        CellSelection cell = newWorksheet.Cells[row, item.Level];
        cell.SetValue(item.Text);

        cell = newWorksheet.Cells[row++, 2];
        cell.SetValue(item.Checked);

    }

    var formatProvider = new XlsxFormatProvider();


    var bytes = formatProvider.Export(workbook);
    File.WriteAllBytes(@"D:\Test.xlsx", bytes);


}

To reproduce: please refer to the attached sample project and try to reorder a node. You will notice that the message box is not rendered properly.

        //case 2
        private void DragDropService_PreviewDragDrop(object sender, RadDropEventArgs e)
        {
             e.Handled = true;
            RadMessageBox.Show(this, "Showing a messagebox under dragending doesn't work correctly.");
        }

        //case 1
        private void radTreeView1_DragEnding(object sender, Telerik.WinControls.UI.RadTreeViewDragCancelEventArgs e)
        {
            //e.Cancel = true;
            //RadMessageBox.Show(this, "Showing a messagebox under dragending doesn't work correctly.");
        }

Workaround: subscribe to the TreeViewElement.DragDropService.PreviewDragDrop and set the Handled argument to true if you want to cancel the drop operation. Then, handle the TreeViewElement.DragDropService.Stopped event and show the desired message.

        public Form1()
        {
            InitializeComponent();
            
            this.radTreeView1.TreeViewElement.DragDropService.PreviewDragDrop += DragDropService_PreviewDragDrop;
            this.radTreeView1.TreeViewElement.DragDropService.Stopped+=DragDropService_Stopped;
        }

        private void DragDropService_Stopped(object sender, EventArgs e)
        {
            
            RadMessageBox.Show(this, "Showing a messagebox under dragending doesn't work correctly.");
        }
 
        private void DragDropService_PreviewDragDrop(object sender, RadDropEventArgs e)
        {
            e.Handled = true;
        }

To reproduce:

Run the attached application. Drag Node 2 to after Node 4 while holding down the Alt key. Note that the Node 2 copy is inserted before Node 4.

When Node 2 is dragged to after Node 4 without holding down the Alt key, Node 2 is correctly moved to after Node 4.

Workaround: you can modify the TreeViewDragDropService and control at what position exactly to be inserted the dragged node:

https://docs.telerik.com/devtools/winforms/treeview/drag-and-drop/modify-the-dragdropservice-behavior
https://docs.telerik.com/devtools/winforms/treeview/drag-and-drop/drag-and-drop-in-bound-mode

To reproduce: add a RadTreeView and a RadBreadCrumb and apply the MaterialTeal theme. Refer to the attached screenshot illustrating the wrong font of the selected item.

Workaround:

    public partial class RadForm1 : Telerik.WinControls.UI.RadForm
    {
        public RadForm1()
        {
            InitializeComponent(); 

            ThemeResolutionService.ApplicationThemeName = "MaterialTeal";

            this.radTreeView1.NodeFormatting += radTreeView1_NodeFormatting;
            this.radTreeView1.SelectedNodeChanged += radTreeView1_SelectedNodeChanged; 
        }
 
        private void UpdateFont()
        {
            foreach (RadSplitButtonElement item in this.radBreadCrumb1.BreadCrumbElement.Items)
            {
                foreach (RadMenuItem menuItem in item.Items)
                {
                    if (this.radTreeView1.SelectedNode != null && menuItem.Text == this.radTreeView1.SelectedNode.Text)
                    {
                        menuItem.Font = new Font(f.FontFamily,f.Size, FontStyle.Bold);
                    }
                }
            }
        }
        
        private void radTreeView1_SelectedNodeChanged(object sender, Telerik.WinControls.UI.RadTreeViewEventArgs e)
        {
            UpdateFont();
        }

        Font f = null;

        private void radTreeView1_NodeFormatting(object sender, Telerik.WinControls.UI.TreeNodeFormattingEventArgs e)
        {
            f = e.NodeElement.ContentElement.Font;
        }