private RadWorksheetEditor activeWorksheetEditor;

public MainWindow()
{
  InitializeComponent();

  this.spreadsheet.ActiveSheetEditorChanged += (s, e) =>
  {
 if (activeWorksheetEditor != null)
 {
 activeWorksheetEditor.Selection.SelectionChanged -= Selection_SelectionChanged;
 }

 activeWorksheetEditor = this.spreadsheet.ActiveWorksheetEditor;
 activeWorksheetEditor.Selection.SelectionChanged += Selection_SelectionChanged;
  };
}

private void Selection_SelectionChanged(object? sender, EventArgs e)
{
  this.spreadsheet.ActiveWorksheetEditor.Selection.FillSelection.IsEnabled = true;
}

public class CustomMouseSelectionHandler : MouseSelectionHandler
{
    private IDocumentEditorPresenter presenter;
    private RadDocument document;

    public CustomMouseSelectionHandler(RadDocument document, IDocumentEditorPresenter presenter) 
        : base(document, presenter)
    {
        this.presenter = presenter;
        this.document = document;
    }      

    public override void RegisterDocumentMouseDown(bool ctrlPressed, bool shiftPressed, Point position, UIElement originalSource = null, SourceType source = SourceType.Mouse)
    {
        base.RegisterDocumentMouseDown(ctrlPressed, shiftPressed, position, originalSource, source);
        var box = this.document.CaretPosition.GetCurrentSpanBox();
        if (box != null && box.AssociatedSpan.Text == "¬")
        {
            document.CaretPosition.MoveToCurrentLineEnd();
        }
    }
}

 this.richTextBox.Loaded += (s, args) =>
 {
     var presenter = (DocumentPresenterBase)this.richTextBox.ActiveEditorPresenter;
     presenter.MouseSelectionHandler = new CustomMouseSelectionHandler(this.richTextBox.Document, presenter);
 };

<Window x:Class="RadHighlightTextBlock_Issue.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:local="clr-namespace:RadHighlightTextBlock_Issue"
        xmlns:telerik="http://schemas.telerik.com/2008/xaml/presentation"
        mc:Ignorable="d"
        Title="MainWindow" Height="450" Width="800">
    <Window.Resources>
        <local:MyViewModel x:Key="MyViewModel"/>
    </Window.Resources>
    <StackPanel>
        <telerik:RadHighlightTextBlock x:Name="highlightTextBlock" 
                                       Text="{Binding Source={StaticResource MyViewModel}, Path=SelectedItem.Description}"
                                      HighlightText="{Binding Source={StaticResource MyViewModel}, Path=SelectedItem.HighlightText}"
                                      HighlightForeground="Red"
                                      HighlightBackground="Yellow"
                                      FontSize="16"
                                      Margin="10"/>
        <telerik:RadButton Content="Content0" Command="{Binding Source={StaticResource MyViewModel}, Path=C0Command}"/>
        <telerik:RadButton Content="Content1" Command="{Binding Source={StaticResource MyViewModel}, Path=C1Command}"/>
    </StackPanel>
</Window>

using System;
using System.Collections.ObjectModel;
using System.ComponentModel;
using System.Linq;
using System.Runtime.CompilerServices;
using System.Windows;
using System.Windows.Input;

namespace RadHighlightTextBlock_Issue
{
    /// <summary>
    /// MainWindow.xaml 的互動邏輯
    /// </summary>
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
        }

        private void RadButton_Click(object sender, RoutedEventArgs e)
        {
            highlightTextBlock.Text = "This is a sample text to demonstrate the RadHighlightTextBlock control.";
        }

        private void RadButton_Click_1(object sender, RoutedEventArgs e)
        {
            highlightTextBlock.Text = "This is another sample text for the second button.";
        }
    }

    public class MyViewModel : INotifyPropertyChanged
    {
        public ObservableCollection<DataItem> Items { get; set; }
        public MyViewModel()
        {
            Items = new ObservableCollection<DataItem>
            {
                new DataItem { HighlightText = "sample", Description = "This is a sample description." },
                new DataItem { HighlightText = "second", Description = "This is the second item description." },
                new DataItem { HighlightText = "text", Description = "This item contains the word text." }
            };

            SelectedItem = Items.FirstOrDefault();
        }

        public ICommand C0Command => new RelayCommand<object>((obj) =>
        {
            SelectedItem = Items.FirstOrDefault();
        });

        public ICommand C1Command => new RelayCommand<object>((obj) =>
        {
            SelectedItem = Items.LastOrDefault();
        });

        private DataItem _selectedItem;
        public DataItem SelectedItem
        {
            get { return _selectedItem; }
            set
            {
                if (_selectedItem != value)
                {
                    _selectedItem = value;
                    OnPropertyChanged();
                }
            }
        }

        public event PropertyChangedEventHandler PropertyChanged;

        protected virtual void OnPropertyChanged([CallerMemberName] string propertyName = "")
        {
            var handler = PropertyChanged;
            if (handler != null)
            {
                handler(this, new PropertyChangedEventArgs(propertyName));
            }
        }
    }

    public class DataItem
    {
        public string HighlightText { get; set; }
        public string Description { get; set; }
    }

    /*Implement RelayCommand...*/
}

System.ArgumentException: 'Requested distance is outside the content of the associated document.'PresentationFramework.dll!System.Windows.Documents.TextPointer.TextPointer(System.Windows.Documents.TextContainer textContainer, int offset, System.Windows.Documents.LogicalDirection direction)
  PresentationFramework.dll!System.Windows.Documents.TextContainer.CreatePointerAtOffset(int offset, System.Windows.Documents.LogicalDirection direction) 
  PresentationFramework.dll!System.Windows.Documents.TextParentUndoUnit.Do() 
  PresentationFramework.dll!MS.Internal.Documents.UndoManager.Redo(int count) 
  PresentationFramework.dll!System.Windows.Documents.TextStore.RedoQuietly(int count)  PresentationFramework.dll!System.Windows.Documents.TextStore.SetFinalDocumentState(MS.Internal.Documents.UndoManager undoManager, System.Collections.Stack imeChangeStack, int appChangeCount, int imeSelectionAnchorOffset, int imeSelectionMovingOffset, int appSelectionAnchorOffset, int appSelectionMovingOffset)  PresentationFramework.dll!System.Windows.Documents.TextStore.HandleCompositionEvents(int previousUndoCount)  PresentationFramework.dll!System.Windows.Documents.TextStore.GrantLockWorker(MS.Win32.UnsafeNativeMethods.LockFlags flags)   PresentationFramework.dll!System.Windows.Documents.TextStore.RequestLock(MS.Win32.UnsafeNativeMethods.LockFlags flags, out int hrSession)

FormatException: The input string '4514.5' was not in the correct format.
at System.Single.Parse(String s)
Telerik.Windows.Controls.RichTextBox.dll!Telerik.Windows.Documents.FormatProviders.OpenXml.Docx.Import.TableImporter.ImportTableGrid(Telerik.Windows.Documents.FormatProviders.OpenXml.Docx.Parsing.Style style)  Telerik.Windows.Controls.RichTextBox.dll!Telerik.Windows.Documents.FormatProviders.OpenXml.Docx.Import.TableImporter.Import(Telerik.Windows.Documents.FormatProviders.OpenXml.Docx.Parsing.Style parentStyle)  Telerik.Windows.Controls.RichTextBox.dll!Telerik.Windows.Documents.FormatProviders.OpenXml.Docx.Import.MainDocumentImporter.BuildTable(Telerik.Windows.Documents.FormatProviders.OpenXml.Docx.Parsing.Style parentStyle)  Telerik.Windows.Controls.RichTextBox.dll!Telerik.Windows.Documents.FormatProviders.OpenXml.Docx.Import.MainDocumentImporter.BuildBody()  Telerik.Windows.Controls.RichTextBox.dll!Telerik.Windows.Documents.FormatProviders.OpenXml.Docx.Import.MainDocumentImporter.BuildDocument()  Telerik.Windows.Controls.RichTextBox.dll!Telerik.Windows.Documents.FormatProviders.OpenXml.Docx.Import.MainDocumentImporter.Import()Telerik.Windows.Controls.RichTextBox.dll!Telerik.Windows.Documents.FormatProviders.OpenXml.Docx.Import.DocxImporter.ReadXmlContentFromPackage(Telerik.Windows.Documents.FormatProviders.OpenXml.Docx.Import.DocxPartImporterBase importer)  Telerik.Windows.Controls.RichTextBox.dll!Telerik.Windows.Documents.FormatProviders.OpenXml.Docx.Import.DocxImporter.ReadXmlContentAndRelationsFromPackage(Telerik.Windows.Documents.FormatProviders.OpenXml.Docx.Import.DocxPartImporterBase importer)
Telerik.Windows.Controls.RichTextBox.dll!Telerik.Windows.Documents.FormatProviders.OpenXml.Docx.Import.DocxImporter.Import()  Telerik.Windows.Controls.RichTextBox.dll!Telerik.Windows.Documents.FormatProviders.OpenXml.Docx.DocxFormatProvider.Import(System.IO.Stream input) 

 var cultureCache = Thread.CurrentThread.CurrentCulture;
 Thread.CurrentThread.CurrentCulture = System.Globalization.CultureInfo.InvariantCulture;
 Thread.CurrentThread.CurrentUICulture = System.Globalization.CultureInfo.InvariantCulture;
 var provider= new DocxFormatProvider();
 rtb.Document = provider.Import(stream);
 Thread.CurrentThread.CurrentCulture = cultureCache;
 Thread.CurrentThread.CurrentUICulture = cultureCache;

InvalidOperationException
   at System.ThrowHelper.ThrowInvalidOperationException_InvalidOperation_EnumFailedVersion() in /_/src/libraries/System.Private.CoreLib/src/System/ThrowHelper.cs:line 432
   at System.Collections.Generic.List`1.Enumerator.MoveNext() in /_/src/libraries/System.Private.CoreLib/src/System/Collections/Generic/List.cs:line 1130
   at System.Linq.Enumerable.SelectEnumerableIterator`2.MoveNext()
   at Telerik.Windows.Data.EnumerableExtensions.<SelectRecursive>d__15`1.MoveNext()
   at System.Linq.Enumerable.SelectEnumerableIterator`2.MoveNext()
   at Telerik.Windows.Data.HierarchicalCollectionViewBase.PopulateInternalList(IQueryable view)
   at Telerik.Windows.Data.QueryableCollectionView.get_InternalList()
   at Telerik.Windows.Data.HierarchicalCollectionViewBase.get_InternalCount()
   at Telerik.Windows.Data.DataItemCollection.get_Item(Int32 index)
   at Telerik.Windows.Controls.GridView.Rows.GetRowItemsAtRange(Int32 startIndex, Int32 endIndex)
   at Telerik.Windows.Controls.GridView.GridViewVirtualizingPanel.FlatLayoutStrategy.RealizeRows(Int32 startIndex, Int32 endIndex, Double& verticalOffset, HashSet`1& realizedRows)
   at Telerik.Windows.Controls.GridView.GridViewVirtualizingPanel.FlatLayoutStrategy.MeasureOverride(Size availableSize)
   at Telerik.Windows.Controls.GridView.GridViewVirtualizingPanel.MeasureOverride(Size availableSize)

NullReferenceException
   at Telerik.Windows.Data.QueryableCollectionView.InternalGetItemAt(Int32 index) in Telerik.Windows.Data\QueryableCollectionView.cs:line 3081
   at Telerik.Windows.Controls.GridView.Rows.GetRowItemsAtRange(Int32 startIndex, Int32 endIndex)
   at Telerik.Windows.Controls.GridView.GridViewVirtualizingPanel.FlatLayoutStrategy.RealizeRows(Int32 startIndex, Int32 endIndex, Double& verticalOffset, HashSet`1& realizedRows)
   at Telerik.Windows.Controls.GridView.GridViewVirtualizingPanel.FlatLayoutStrategy.MeasureOverride(Size availableSize)
   at Telerik.Windows.Controls.GridView.GridViewVirtualizingPanel.MeasureOverride(Size availableSize)

 Dispatcher.BeginInvoke(new Action(() =>
 {
    // add/remove items from the source collection
 }));