Use the attached project to reproduce.
- Check on of the nodes and then press Alt +T (do not move the mouse away from the node)

Workaround:
Set transparent fill and border to the TreeNodeElement disabled state in Visual Style Builder.

If the ShowLines property is set to true and a node's expander is hidded, a link line should appear instead.
Comment: When using full lazy mode RadGridView displays expander icon for all nodes. When it is hidden there is a whitespace. You can show tree lines only with code.

To reproduce: select a node and click the button. Note: it may be necessary to perform this several times.

public partial class Form1 : Form
{
    public Form1()
    {
        InitializeComponent();
    }

    private List<DataObject> GetDataList()
    {
        var ret = new List<DataObject>();

        ret.Add(new DataObject() { Node = "A", Parent = null });
        ret.Add(new DataObject() { Node = "B", Parent = "A" });
        ret.Add(new DataObject() { Node = "C", Parent = "B" });
        ret.Add(new DataObject() { Node = "D", Parent = "B" });

        return ret;
    }
    
    private void button1_Click(object sender, EventArgs e)
    {
        radTreeView1.Nodes.Clear(); 
        radTreeView1.DisplayMember = "Node";
        radTreeView1.ChildMember = "Node";
        radTreeView1.ParentMember = "Parent";
        radTreeView1.DataSource = GetDataList();
        radTreeView1.ExpandAll();
    }
}

public class DataObject
{
    public string Node { get; set; }

    public string Parent { get; set; }
}

Workaround: instead of clearing the nodes, set the DataSource property to null.

If ALT key is pressed, the dragged nodes should be copied when drag and drop is performed.

Workaround: 
this.radTreeView1.TreeViewElement.InvalidateMeasure();
this.radTreeView1.TreeViewElement.InvalidateArrange();

To reproduce:
- Open the attached project.
- Scroll to the right.
- Drag a node.
- The DropHint line is outside the window.
 
Workaround:
class CustomTreeView : RadTreeView
{
    //Replace the default element with the custom one
    protected override RadTreeViewElement CreateTreeViewElement()
    {
        return new CustomTreeViewElement();
    }
    //Enable theming for the control
    public override string ThemeClassName
    {
        get
        {
            return typeof(RadTreeView).FullName;
        }
    }
}
class CustomTreeViewElement : RadTreeViewElement
{
    //Enable themeing for the element
    protected override Type ThemeEffectiveType
    {
        get
        {
            return typeof(RadTreeViewElement);
        }
    }
    //Replace the default drag drop service with the custom one
    protected override TreeViewDragDropService CreateDragDropService()
    {
        return new MyDragDropService(this);
    }
}
class MyDragDropService : TreeViewDragDropService
{
    public MyDragDropService(RadTreeViewElement owner)
    : base(owner)
{ }
    protected override void UpdateHintPosition(Point mousePosition)
    {
        base.UpdateHintPosition(mousePosition);
        RadLayeredWindow dropHint =typeof(TreeViewDragDropService).GetField("dropHintWindow", System.Reflection.BindingFlags.NonPublic | System.Reflection.BindingFlags.Instance).GetValue(this) as RadLayeredWindow;
        if (dropHint != null)
        {
            TreeNodeElement nodeElement = this.DropTarget as TreeNodeElement;
           
            dropHint.Location = new Point (nodeElement.ElementTree.Control.PointToScreen(Point.Empty).X, dropHint.Location.Y);
        }
    }
}

If the RadTreeView does not have node, the Property Builder is closed when the Color Selector button is clicked.

You cannot unsubscribe from the NodeCheckedChanging event of RadTreeView.

The AllowDrop property of RadTreeNode allows dropping of node when its value is false.

To reproduce:
1) Create a Windows Forms project.
2) Add a RadTreeView control.
3) Set the KeyboardSearchEnabled property to true.
4) Run the application and click in the tree (just to make sure it has the focus).
5) Using the keyboard, press a letter key.
6) Minimize the application.

The same exception occurs if there are no selected nodes.

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

        set
        {
            base.ThemeClassName = value;
        }
    }
}
class MyTreeViewElement : RadTreeViewElement
{
    protected override Type ThemeEffectiveType
    {
        get
        {
            return typeof(RadTreeViewElement);
        }
    }
    protected override RadTreeNode GetFirstMatch(string searchCriteria, RadTreeNodeCollection nodes)
    {
        if(this.SelectedNode == null && this.Items == null)
        {
            return null;
        }
        return base.GetFirstMatch(searchCriteria, nodes);
    }
}

The check boxes are squeezed when the expanded state of the child nodes is changed in NodeCheckedChanged event.

Possible workaround: use Begin-End update block 

How to reproduce:
public partial class RadForm1 : Telerik.WinControls.UI.RadForm
{
    private BindingList<TreeViewDataObject> data;
    public static int Id = 0;

    public RadForm1()
    {
        InitializeComponent();

        var theme = new FluentTheme();
        ThemeResolutionService.ApplicationThemeName = theme.ThemeName;

        this.data = new BindingList<TreeViewDataObject>();


        for (int i = 1; i <= 5; i += 1)
        {
            Id++;
            TreeViewDataObject root = new TreeViewDataObject()
            {
                Id = Id,
                ParentId = -1,
                Name = "Root: " + i
            };

            this.data.Add(root);

            for (int j = 1; j <= 3; j++)
            {
                Id++;
                TreeViewDataObject child = new TreeViewDataObject()
                {
                    Id = Id,
                    ParentId = root.Id,
                    Name = "Child: " + Id
                };

                this.data.Add(child);

                for (int K = 1; K <= 5; K++)
                {
                    Id++;

                    TreeViewDataObject c = new TreeViewDataObject()
                    {
                        Id = Id,
                        ParentId = child.Id,
                        Name = "Child: " + Id
                    };

                    this.data.Add(c);
                }
            }
        }

        this.radTreeView1.DataSource = this.data;
        this.radTreeView1.DisplayMember = "Name";
        this.radTreeView1.ParentMember = "ParentId";
        this.radTreeView1.ChildMember = "Id";
        this.radTreeView1.RelationBindings.Add(new RelationBinding(this.data, "Name", "ParentId", "Id"));

        this.radTreeView1.ExpandAll();

        this.radTreeView1.TreeViewElement.AllowEdit = true;
        this.radTreeView1.TreeViewElement.AllowAdd = true;
        this.radTreeView1.TreeViewElement.AllowRemove = true;
    }
}

  public class TreeViewDataObject
    {
        public int Id { get; set; }

        public int ParentId { get; set; }

        public string Name { get; set; }
    }

Workaround: Handle the ContextMenuOpening event and add logic for creating a new record in the data source object
public partial class RadForm1 : Telerik.WinControls.UI.RadForm
{
    private BindingList<TreeViewDataObject> data;
    private int id = 0;

    public RadForm1()
    {
        InitializeComponent();

        var theme = new FluentTheme();
        ThemeResolutionService.ApplicationThemeName = theme.ThemeName;

        this.data = new BindingList<TreeViewDataObject>();


        for (int i = 1; i <= 5; i += 1)
        {
            id++;
            TreeViewDataObject root = new TreeViewDataObject()
            {
                Id = id,
                ParentId = -1,
                Name = "Root: " + i
            };

            this.data.Add(root);

            for (int j = 1; j <= 3; j++)
            {
                id++;
                TreeViewDataObject child = new TreeViewDataObject()
                {
                    Id = id,
                    ParentId = root.Id,
                    Name = "Child: " + id
                };

                this.data.Add(child);

                for (int K = 1; K <= 5; K++)
                {
                    id++;

                    TreeViewDataObject c = new TreeViewDataObject()
                    {
                        Id = id,
                        ParentId = child.Id,
                        Name = "Child: " + id
                    };

                    this.data.Add(c);
                }
            }
        }

        this.radTreeView1.DataSource = this.data;
        this.radTreeView1.DisplayMember = "Name";
        this.radTreeView1.ParentMember = "ParentId";
        this.radTreeView1.ChildMember = "id";
        this.radTreeView1.RelationBindings.Add(new RelationBinding(this.data, "Name", "ParentId", "id"));

        this.radTreeView1.ExpandAll();
        
        this.radTreeView1.ContextMenuOpening += RadTreeView1_ContextMenuOpening;

        this.radTreeView1.TreeViewElement.AllowEdit = true;
        this.radTreeView1.TreeViewElement.AllowAdd = true;
        this.radTreeView1.TreeViewElement.AllowRemove = true;
    }

    private void RadTreeView1_ContextMenuOpening(object sender, TreeViewContextMenuOpeningEventArgs e)
    {
        TreeViewDefaultContextMenu treeMenu = e.Menu as TreeViewDefaultContextMenu;
        treeMenu.Items.Remove(treeMenu.AddMenuItem);

        RadMenuItem item = new RadMenuItem("&New");

        item.Click -= Item_Click;
        item.Click += Item_Click;
        treeMenu.Items.Insert(2, item);
    }

    private void Item_Click(object sender, EventArgs e)
    {
        RadTreeNode parent = this.radTreeView1.SelectedNode;
        if (parent == null || parent.DataBoundItem == null)
        {
            return;
        }

        this.id++;
        BindingList<TreeViewDataObject> data = this.radTreeView1.DataSource as BindingList<TreeViewDataObject>;
        TreeViewDataObject newObject = new TreeViewDataObject()
        {
            Id = this.id,
            ParentId = ((TreeViewDataObject)parent.DataBoundItem).Id,
            Name = "New Name"
        };

        parent.Expanded = true;
        data.Add(newObject);
    }
}

public class TreeViewDataObject
{
    public int Id { get; set; }

    public int ParentId { get; set; }

    public string Name { get; set; }
}

To reproduce: add several nodes at design time in order to obtain a vertical scrollbar. Enable the drag and drop functionality by setting the AllowDragDrop property to true. If you start dragging you will notice that the RadTreeView doesn't auto-scroll  when you drag close to the borders of the visible area. 

Workaround: 

    Class CustomDragDropService
        Inherits TreeViewDragDropService

        Public Sub New(ByVal owner As RadTreeViewElement)
            MyBase.New(owner)
        End Sub

        Protected Overrides Sub SetHintWindowPosition(ByVal mousePt As Point)
        End Sub
    End Class

    Class CustomTreeViewElement
        Inherits RadTreeViewElement

        Protected Overrides ReadOnly Property ThemeEffectiveType As Type
            Get
                Return GetType(RadTreeViewElement)
            End Get
        End Property

        Protected Overrides Function CreateDragDropService() As TreeViewDragDropService
            Return New CustomDragDropService(Me)
        End Function
    End Class

    Class CustomTreeView
        Inherits RadTreeView

        Protected Overrides Function CreateTreeViewElement() As RadTreeViewElement
            Return New CustomTreeViewElement()
        End Function

        Public Overrides Property ThemeClassName As String
            Get
                Return GetType(RadTreeView).FullName
            End Get
            Set(value As String)
                MyBase.ThemeClassName = value
            End Set
        End Property
    End Class

When you open the last node for edit programmatically, either an exception is thrown or the editor is not opened.

Steps to reproduce:

1. Drag a RadTreeView from Toolbox and drop it onto the form.
2. Set the size of RadTreeView at design time to Size(354, 248).
3. Add a RadButton.
4. Use the following code:
public Form1()
{
    InitializeComponent(); 
    radTreeView1.AllowEdit = true; 
    radTreeView1.Nodes.Add("Root");
    for (int i = 0; i < 20; i++)
    {
        radTreeView1.Nodes[0].Nodes.Add("Test" + i.ToString());
    }
}

private void radButton1_Click(object sender, EventArgs e)
{
    RadTreeNode node = new RadTreeNode("Demo" + radTreeView1.Nodes[0].Nodes.Count.ToString());
    radTreeView1.Nodes[0].Nodes.Add(node);
    node.BeginEdit();
}

5. Click the button.
6. Collapse the root node.
7. Click the button again.



Workaround:

        private void button1_Click(object sender, EventArgs e)
        {
            RadTreeNode node = new RadTreeNode("Demo" + radTreeView1.Nodes[0].Nodes.Count.ToString());
            radTreeView1.Nodes[0].Nodes.Add(node);
            this.radTreeView1.TreeViewElement.Update(RadTreeViewElement.UpdateActions.Reset);
            node.Parent.Expanded = true;
            ((MyTreeViewElement)this.radTreeView1.TreeViewElement).SuspendEndEdit = true;
            node.BeginEdit();
            ((MyTreeViewElement)this.radTreeView1.TreeViewElement).SuspendEndEdit = false;
        } 

    public class MyTreeView : RadTreeView
    {
        protected override RadTreeViewElement CreateTreeViewElement()
        {
            return new MyTreeViewElement();
        }

        public override string ThemeClassName
        {
            get
            {
                return typeof(RadTreeView).FullName;
            }
            set
            {
            }
        }
    }

    public class MyTreeViewElement : RadTreeViewElement
    {
        public bool SuspendEndEdit = false;

        protected override Type ThemeEffectiveType
        {
            get
            {
                return typeof(RadTreeViewElement);
            }
        }

        protected override bool EndEditCore(bool commitChanges)
        {
            if (SuspendEndEdit)
            {
                return false;
            }

            return base.EndEditCore(commitChanges);
        }
    }

To reproduce:
public Form1()
{
    InitializeComponent(); 
    this.radTreeView1.NodesNeeded+=radTreeView1_NodesNeeded;
}

private void radTreeView1_NodesNeeded(object sender, Telerik.WinControls.UI.NodesNeededEventArgs e)
{
    Console.WriteLine("NodesNeeded"); 
}

As a result StackOverFlowException is thrown.

Workaround: Do not subscribe to the NodesNeeded event if the LazyMode property is set to false.

If the DPI configuration has 120% value, Save, Close and Cancel buttons disappear from the Property Builder form.

To reproduce:

public BlankTreeView()
{
    InitializeComponent();
    RadContextMenu menu = new RadContextMenu();
    RadMenuItem addItem = new RadMenuItem("Add");
    addItem.Click += addItem_Click;
    menu.Items.Add(addItem);
    this.radTreeView1.RadContextMenu = menu;
    
    BindingList<ParentObject> items = new BindingList<ParentObject>();
    for (int i = 0; i < 3; i++)
    {
        BindingList<ChildObject> subItems = new BindingList<ChildObject>();
        for (int j = 0; j < 10; j++)
        {
            subItems.Add(new ChildObject(j,"SubNode" + i + "." + j));
        }

        items.Add(new ParentObject(i ,"Node" + i,subItems));
    }

    this.radTreeView1.DataSource = items;
    this.radTreeView1.DisplayMember = "Title\\Description";
    this.radTreeView1.ChildMember = "ParentObject\\Children";
}

private void addItem_Click(object sender, EventArgs e)
{
    if (this.radTreeView1.SelectedNode != null)
    {
        if (this.radTreeView1.SelectedNode.Level == 0)
        {
            Random rand = new Random(); 
            ParentObject dataItem = this.radTreeView1.SelectedNode.DataBoundItem as ParentObject;
            if (dataItem != null)
            {
                dataItem.Children.Add(new ChildObject(rand.Next(0, 100),"New Node"));
            } 
        }
    }
}

public class ParentObject: INotifyPropertyChanged
{
    public event PropertyChangedEventHandler PropertyChanged;

    protected virtual void OnPropertyChanged(string propertyName)
    {
        if (PropertyChanged != null)
        {
            PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
        }
    }
    
    private int id;
    private string title;
    private BindingList<ChildObject> children;

    public int ID
    {
        get
        {
            return this.id;
        }
        set
        {
            this.id = value;
            OnPropertyChanged("ID");
        }
    }

    public string Title
    {
        get
        {
            return this.title;
        }
        set
        {
            this.title = value;
            OnPropertyChanged("Title");
        }
    }

    public BindingList<ChildObject> Children
    {
        get
        {
            return this.children;
        }
        set
        {
            this.children = value;
            OnPropertyChanged("Children");
        }
    }

    public ParentObject(int iD, string title, BindingList<ChildObject> children)
    {
        this.ID = iD;
        this.Title = title;
        this.Children = children;
    }
}

public class ChildObject: INotifyPropertyChanged
{ 
    public event PropertyChangedEventHandler PropertyChanged;

    private int id;
    private string description;

    public int ID
    {
        get
        {
            return this.id;
        }
        set
        {
            this.id = value;
            OnPropertyChanged("ID");
        }
    }

    public string Description
    {
        get
        {
            return this.description;
        }
        set
        {
            this.description = value;
            OnPropertyChanged("Description");
        }
    }

    public ChildObject(int iD, string description)
    {
        this.id = iD;
        this.description = description;
    }

    protected virtual void OnPropertyChanged(string propertyName)
    {
        if (PropertyChanged != null)
        {
            PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
        }
    }
}

Workaround: reset the TreeViewElement after adding the record:

radTreeView1.TreeViewElement.Update(RadTreeViewElement.UpdateActions.Reset);

To reproduce:

1.Create new RadTreeView

2.Create new ImageList and populate

3.Assign ImageList to RadTreeView

4.Open Property Builder

5.Assign Image to a Node, Apply, Close

6.Re-Open, change Node to a different Image, the previous image remains

Expected: the new image is successfully updated.

Actual: the old image remains: 

If the list separator for the culture is ";"

and you have nodes in RadTreeView that are bold, the following XML will be serialized:

var xml = "<TreeView xmlns:xsi=\"http://www.w3.org/2001/XMLSchema-instance\" xmlns:xsd=\"http://www.w3.org/2001/XMLSchema\" MultiSelect=\"true\" AllowDragDrop=\"true\" LabelEdit=\"true\" AllowDrop=\"true\">  <Nodes Text=\"New Package\" Expanded=\"true\" ImageKey=\"Package\" Font=\"Microsoft Sans Serif; 8,25pt; style=Bold\">    <Tag xsi:type=\"xsd:string\">&lt;Info&gt;&lt;FullName&gt;\\New Package&lt;/FullName&gt;&lt;ObjectType&gt;Package&lt;/ObjectType&gt;&lt;IsRunnable&gt;False&lt;/IsRunnable&gt;&lt;IsContainer&gt;true&lt;/IsContainer&gt;&lt;/Info&gt;</Tag>    <Nodes Text=\"Query Engine\" ImageKey=\"QueryEngine\" Expanded=\"true\" Font=\"Microsoft Sans Serif; 8,25pt; style=Bold\">      <Tag xsi:type=\"xsd:string\">&lt;Info&gt;&lt;FullName&gt;\\Query Engine&lt;/FullName&gt;&lt;ObjectType&gt;QueryEngine&lt;/ObjectType&gt;&lt;IsRunnable&gt;true&lt;/IsRunnable&gt;&lt;IsTableEntity&gt;true&lt;/IsTableEntity&gt;&lt;IsCut&gt;false&lt;/IsCut&gt;&lt;/Info&gt;</Tag>    </Nodes>    <Nodes Text=\"Connector\" ImageKey=\"Irion.SQLServer\">      <Tag xsi:type=\"xsd:string\">&lt;Info&gt;&lt;FullName&gt;\\Connector&lt;/FullName&gt;&lt;ObjectType&gt;DBConnection&lt;/ObjectType&gt;&lt;IsCut&gt;false&lt;/IsCut&gt;&lt;/Info&gt;</Tag>    </Nodes>    <Nodes Text=\"Connector Link\" ImageKey=\"DatabaseDatalink\">      <Tag xsi:type=\"xsd:string\">&lt;Info&gt;&lt;FullName&gt;\\Connector Link&lt;/FullName&gt;&lt;ObjectType&gt;DatabaseDatalink&lt;/ObjectType&gt;&lt;IsRunnable&gt;true&lt;/IsRunnable&gt;&lt;IsTableEntity&gt;true&lt;/IsTableEntity&gt;&lt;IsCut&gt;false&lt;/IsCut&gt;&lt;/Info&gt;</Tag>    </Nodes>  </Nodes></TreeView>";

However, if you try to load this layout on another machine where the list separator is "," the nodes wouldn't be bold:

Expected:

Actual:

Use the following code snippet and compare the filtering performance when using bound and unbound mode:

         public RadForm1()
        {
            InitializeComponent(); 

            this.radTextBox1.TextChanged += this.RadTextBox1_TextChanged;
             this.radTextBox2.TextChanged += this.RadTextBox2_TextChanged;
            List<Data> list = new List<Data>();
            this.radTreeView1.BeginUpdate();
            for (int i = 0; i < 100000; i++)
            {
                list.Add(new Data()
                {
                    Id = i,
                    Name = "MyData_"+i,
                    ParentId = -1
                }) ;

                this.radTreeView1.Nodes.Add("MyData_"+i);
            }
            this.radTreeView1.EndUpdate();
            this.radTreeView2.DisplayMember = "Name";
            this.radTreeView2.ParentMember = "ParentId";
            this.radTreeView2.DataSource = list; 
        }

         private void RadTextBox2_TextChanged(object sender, EventArgs e)
         {
               this.radTreeView2.Filter = this.radTextBox2.Text;
         } 
        private void RadTextBox1_TextChanged(object sender, EventArgs e)
        {
            this.radTreeView1.Filter = this.radTextBox1.Text;
        }
    }

    public class Data
    {
        public int Id { get; set; }
        public string Name { get; set; } 
        public int ParentId { get; set; }
    }

Expected behavior: the performance in bound and unbound mode should be quite similar

Actual behavior: the performance is much slower in bound mode