How to reproduce:
public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();

            this.radGridView1.DataSource = this.GetData();
            GridViewComboBoxColumn supplierColumn = new GridViewComboBoxColumn();
            supplierColumn.Name = "SupplierColumn";
            supplierColumn.HeaderText = "Supplier";
            supplierColumn.DataSource = this.GetListItemData(); ;
            supplierColumn.ValueMember = "Id";
            supplierColumn.DisplayMember = "Description";
            supplierColumn.Width = 200;
            this.radGridView1.Columns.Add(supplierColumn);
            this.radGridView1.AutoSizeColumnsMode = Telerik.WinControls.UI.GridViewAutoSizeColumnsMode.Fill;
            this.radGridView1.CellEditorInitialized += RadGridView1_CellEditorInitialized;
        }

        private void RadGridView1_CellEditorInitialized(object sender, GridViewCellEventArgs e)
        {
            RadDropDownListEditor editor = e.ActiveEditor as RadDropDownListEditor;
            if (editor == null)
            {
                return;
            }

            RadDropDownListEditorElement element = (RadDropDownListEditorElement)editor.EditorElement;
            element.ValueChanging -= Element_ValueChanging;
            element.ValueChanging += Element_ValueChanging;
        }

        private void Element_ValueChanging(object sender, ValueChangingEventArgs e)
        {
            Console.WriteLine("ValueChanging");
            Console.WriteLine("Old " + e.OldValue);
            Console.WriteLine("New " + e.NewValue);
        }

        private object GetData()
        {
            DataTable dt = new DataTable();
            dt.Columns.Add("Id", typeof(int));
            dt.Columns.Add("Name", typeof(string));
            dt.Columns.Add("Date", typeof(DateTime));

            for (int i = 0; i < 100; i++)
            {
                dt.Rows.Add(i, "Name " + i, DateTime.Now.AddDays(i));
            }

            return dt;
        }

        public List<Item> GetListItemData()
        {
            List<Item> items = new List<Item>();
            for (int i = 0; i < 10; i++)
            {
                items.Add(new Item(i, "Data" + i, DateTime.Now.AddDays(i).AddHours(i)));
            }

            return items;
        }
    }

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

        public string Description { get; set; }

        public DateTime Date { get; set; }

        public Item(int id, string description, DateTime date)
        {
            this.Id = id;
            this.Description = description;
            this.Date = date;
        }
    }

Workaround: handle the TextChanging event and store locally the old value
 private void RadGridView1_CellEditorInitialized(object sender, GridViewCellEventArgs e)
 {
     RadDropDownListEditor editor = e.ActiveEditor as RadDropDownListEditor;
     if (editor == null)
     {
         return;
     }

     RadDropDownListEditorElement element = (RadDropDownListEditorElement)editor.EditorElement;

     element.TextChanging-= Element_TextChanging;
     element.TextChanging += Element_TextChanging;
     element.ValueChanging -= Element_ValueChanging;
     element.ValueChanging += Element_ValueChanging;
 }

 object oldValue;
 private void Element_TextChanging(object sender, TextChangingEventArgs e)
 {
     oldValue = e.OldValue;
 }

 private void Element_ValueChanging(object sender, ValueChangingEventArgs e)
 {
     Console.WriteLine("ValueChanging");
     Console.WriteLine("Old " + oldValue);
     Console.WriteLine("New " + e.NewValue);
 }

Until released one can format the nodes this way:
private void RadGridView1_FilterPopupRequired(object sender, Telerik.WinControls.UI.FilterPopupRequiredEventArgs e)
{
    if (e.Column.GetType().Name == "GridViewDateTimeColumn")
    {
        RadListFilterPopup popup = new RadListFilterPopup(e.Column, true);
        e.FilterPopup = popup;
        popup.MenuTreeElement.TreeView.NodeFormatting += TreeView_NodeFormatting;
    }
}
 
int monthNumber = -1;
 
private void TreeView_NodeFormatting(object sender, TreeNodeFormattingEventArgs e)
{
    if (e.Node.Level == 2)
    {
        if (int.TryParse(e.Node.Text, out monthNumber))
        {
            e.NodeElement.ContentElement.Text = System.Globalization.CultureInfo.CurrentCulture.DateTimeFormat.GetMonthName(monthNumber);
        }
    }
}

How to reproduce
public partial class Form1 : RadForm
{
    public Form1()
    {
        InitializeComponent();

        GridViewTextBoxColumn textBoxColumn = new GridViewTextBoxColumn();
        textBoxColumn.Name = "Column";
        textBoxColumn.HeaderText = "Column";
        this.radGridView1.MasterTemplate.Columns.Add(textBoxColumn);

        GridViewTextBoxColumn textBoxColumn2 = new GridViewTextBoxColumn();
        textBoxColumn2.Name = "TextBoxColumn2";
        textBoxColumn2.HeaderText = "ReadOnlyColumn";
        this.radGridView1.MasterTemplate.Columns.Add(textBoxColumn2);

        for (int i = 0; i < 10; i++)
        {
            object v = i * 2;
            if (i % 3 == 0)
            {
                v = null;
            }

            this.radGridView1.Rows.Add(new object[] { i, v });
        }

        this.radGridView1.MultiSelect = true;
        foreach (var row in this.radGridView1.Rows)
        {
            foreach (var cell in row.Cells)
            {
                GridViewCellInfo cellInfo = cell as GridViewCellInfo;
                if (cellInfo != null && cellInfo.RowInfo.Index % 3 == 0 && cellInfo.ColumnInfo.Index == 1)
                {
                    cellInfo.ReadOnly = true;
                }
            }
        }
    }

}

Workaround:
public class MyRadGridView : RadGridView
{
    public override string ThemeClassName
    {
        get
        {
            return typeof(RadGridView).FullName;
        }
    }

    protected override RadGridViewElement CreateGridViewElement()
    {
        return new MyRadGridViewElement();
    }
}

internal class MyRadGridViewElement : RadGridViewElement
{
    protected override Type ThemeEffectiveType
    {
        get
        {
            return typeof(RadGridViewElement);
        }
    }

    protected override MasterGridViewTemplate CreateTemplate()
    {
        return new MyMasterGridViewTemplate();
    }
}

internal class MyMasterGridViewTemplate : MasterGridViewTemplate
{
    protected override void PasteDataToRow(List<string> rowData, GridViewRowInfo row)
    {
        {
            int colIndex = this.Owner.CurrentColumn.Index;
            int j = 0;

            while (j < rowData.Count && colIndex < this.CurrentView.ViewTemplate.ColumnCount)
            {
                GridViewColumn col = this.CurrentView.ViewTemplate.Columns[colIndex];
                if (col.IsVisible && !col.ReadOnly && !row.Cells[colIndex].ReadOnly)
                {

                    object value = rowData[j];

                    if (string.IsNullOrEmpty(rowData[j]))
                    {
                        value = null;
                    }
                    else if (this.CurrentView.ViewTemplate.Columns[colIndex].DataType == typeof(string))
                    {
                        GridViewTextBoxColumn textColumn = col as GridViewTextBoxColumn;

                        if (textColumn != null && textColumn.MaxLength > 0)
                        {
                            if (rowData[j].Length > textColumn.MaxLength)
                            {
                                value = rowData[j].Substring(0, textColumn.MaxLength);
                            }
                        }
                    }
                    else if (this.CurrentView.ViewTemplate.Columns[colIndex].DataType == typeof(DateTime))
                    {
                        try
                        {
                            value = DateTime.Parse(rowData[j], this.CurrentView.ViewTemplate.Columns[colIndex].FormatInfo);
                        }
                        catch { }
                    }
                    else if (this.CurrentView.ViewTemplate.Columns[colIndex].DataType == typeof(Color))
                    {
                        try
                        {
                            value = ColorTranslator.FromHtml(rowData[j]);
                        }
                        catch { }
                    }

                    if (this.ClipboardPasteMode == GridViewClipboardPasteMode.EnableWithNotifications)
                    {
                        CellValidatingEventArgs cellValidating = new CellValidatingEventArgs(row, col, value, row.Cells[colIndex].Value, null);
                        this.EventDispatcher.RaiseEvent<CellValidatingEventArgs>(EventDispatcher.CellValidating, this, cellValidating);

                        if (!cellValidating.Cancel)
                        {
                            row.Cells[colIndex].Value = value;

                            CellValidatedEventArgs cellValidated = new CellValidatedEventArgs(row, col, value);
                            this.EventDispatcher.RaiseEvent<CellValidatedEventArgs>(EventDispatcher.CellValidated, this, cellValidated);
                        }
                    }
                    else
                    {
                        row.Cells[colIndex].Value = value;
                    }

                    j++;
                }

                colIndex++;
            }
        }
    }
}

By default, when I am expanding a group and start sorting, the expanded group doesn't collapse. However, if you use a group comparer, all groups will always collapse, when you click a header cell to sort.

To reproduce: we have a RadPageView with two grids on two pages. The first grid has a ColumnGroupsViewDefinition applied. When you open the Property Builder and try to hide some of the columns, the error occurs.

Workaround: make the changes programamtically at run time.

How to reproduce: check the attached project

Workaround: create a custom RadListFilterPopup
public class MyRadListFilterPopup : RadListFilterPopup
    {
        public MyRadListFilterPopup(GridViewDataColumn dataColumn, bool groupedDateValues)
            : base(dataColumn, groupedDateValues)
        { }

        protected override void OnButtonOkClick(EventArgs e)
        {
            FilterOperator filterOperator = FilterOperator.IsEqualTo;

            IRadListFilterElement listFilterElement = (IRadListFilterElement)typeof(RadListFilterPopup).GetField("listFilterElement", BindingFlags.Instance | BindingFlags.NonPublic).GetValue(this);
            RadListFilterDistinctValuesTable selectedValues = listFilterElement.SelectedValues;

            switch (listFilterElement.SelectedMode)
            {
                case ListFilterSelectedMode.All:
                    filterOperator = FilterOperator.None;
                    break;
                case ListFilterSelectedMode.Null:
                    filterOperator = FilterOperator.IsNull;
                    break;
                case ListFilterSelectedMode.NotNull:
                    filterOperator = FilterOperator.IsNotNull;
                    break;
            }

            if (filterOperator != FilterOperator.IsEqualTo)
            {
                SetFilterOperator(filterOperator);
                this.ClosePopup(RadPopupCloseReason.CloseCalled);
            }
            else
            {
                CompositeFilterDescriptor compositeFilterDescriptor = new CompositeFilterDescriptor();
                compositeFilterDescriptor.PropertyName = this.DataColumn.Name;
                RadListFilterDistinctValuesTable distinctValues = this.GetDistinctValuesTable();
                string blanksKey = RadGridLocalizationProvider.CurrentProvider.GetLocalizedString(RadGridStringId.FilterMenuBlanks);
                bool blanks = selectedValues.Contains(blanksKey);
                if (selectedValues.Count > distinctValues.Count / 2 && !blanks)
                {
                    compositeFilterDescriptor.LogicalOperator = FilterLogicalOperator.And;

                    foreach (DictionaryEntry entry in distinctValues)
                    {
                        string key = (string)entry.Key;
                        if (string.IsNullOrEmpty(key))
                        {
                            key = blanksKey;
                        }

                        if (!selectedValues.Contains(key))
                        {
                            foreach (object value in (ArrayList)entry.Value)
                            {
                                FilterDescriptor descriptor;
                                if (value == DBNull.Value)
                                {
                                    descriptor = new FilterDescriptor(this.DataColumn.Name, FilterOperator.IsNotEqualTo, null);
                                }
                                else if (this.DataColumn is GridViewDateTimeColumn || this.DataColumn.DataType == typeof(DateTime) || this.DataColumn.DataType == typeof(DateTime?))
                                {
                                    descriptor = new DateFilterDescriptor(this.DataColumn.Name, FilterOperator.IsNotEqualTo, (DateTime?)value, false);
                                }
                                else
                                {
                                    descriptor = new FilterDescriptor(this.DataColumn.Name, FilterOperator.IsNotEqualTo, value);
                                }

                                compositeFilterDescriptor.FilterDescriptors.Add(descriptor);
                            }
                        }
                    }
                }
                else
                {
                    compositeFilterDescriptor.LogicalOperator = FilterLogicalOperator.Or;

                    foreach (DictionaryEntry entry in selectedValues)
                    {
                        foreach (object value in (ArrayList)entry.Value)
                        {
                            FilterDescriptor descriptor;
                            if (value == DBNull.Value)
                            {
                                descriptor = new FilterDescriptor(this.DataColumn.Name, FilterOperator.IsNotEqualTo, null);
                            }
                            else if (this.DataColumn is GridViewDateTimeColumn || this.DataColumn.DataType == typeof(DateTime) || this.DataColumn.DataType == typeof(DateTime?))
                            {
                                descriptor = new DateFilterDescriptor(this.DataColumn.Name, FilterOperator.IsEqualTo, (DateTime?)value, false);
                            }
                            else
                            {
                                descriptor = new FilterDescriptor(this.DataColumn.Name, FilterOperator.IsEqualTo, value);
                            }

                            compositeFilterDescriptor.FilterDescriptors.Add(descriptor);
                        }
                    }
                }

                this.FilterDescriptor = compositeFilterDescriptor;
                OnFilterConfirmed();
            }
        }
    }

This new API will be useful for implementing custom sorting scenarios when you need to have custom sorting only for a single column.

A similar API is available for the filtering functionality (MasterTemplate.DataView.FilterEvaluate)

How to reproduce: check the code snippet below and the attached video. 
public partial class Form1 : Form
{
    public Form1()
    {
        InitializeComponent();
        this.SetupGrid();

        this.radGridView1.Dock = DockStyle.Fill;
        this.radGridView1.UseScrollbarsInHierarchy = true;
    }

    private void SetupGrid()
    {
        BindingList<Teacher> teachers = new BindingList<Teacher>();
        BindingList<Student> students = new BindingList<Student>();

        for (int i = 1; i <= 2; i++)
        {
            teachers.Add(new Teacher
            {
                TeacherId = i,
                TeacherFirstName = "FirstName " + i,
                TeacherLastName = "FirstName " + i,

            });

            for (int j = 1; j <= 3; j++)
            {
                students.Add(new Student
                {
                    SudentId = j,
                    TeacherId = i,
                    SudentFirstName = "Student " + j,
                    SudentLastName = "LastName " + j,

                });
            }
        }

        this.radGridView1.Templates.Clear();
        this.radGridView1.DataSource = null;

        this.radGridView1.DataSource = teachers;
        this.radGridView1.AutoSizeColumnsMode = GridViewAutoSizeColumnsMode.Fill;

        GridViewTemplate template = new GridViewTemplate();
        template.AutoSizeColumnsMode = GridViewAutoSizeColumnsMode.Fill;
        template.DataSource = students;
        this.radGridView1.MasterTemplate.Templates.Add(template);

        GridViewRelation relation = new GridViewRelation(radGridView1.MasterTemplate);
        relation.ChildTemplate = template;
        relation.RelationName = "TeacherStudents";
        relation.ParentColumnNames.Add("TeacherId");
        relation.ChildColumnNames.Add("TeacherId");
        this.radGridView1.Relations.Add(relation);

        this.radGridView1.ChildViewExpanded += RadGridView1_ChildViewExpanded1;
    }

    private void RadGridView1_ChildViewExpanded1(object sender, ChildViewExpandedEventArgs e)
    {
    }

    private void radButton1_Click(object sender, EventArgs e)
    {
        this.radGridView1.SaveLayout("..\\..\\save.xml");
    }

    private void radButton2_Click(object sender, EventArgs e)
    {
        this.radGridView1.LoadLayout("..\\..\\save.xml");
    }
}

public class Teacher
{
    public int TeacherId { get; set; }

    public string TeacherFirstName { get; set; }

    public string TeacherLastName { get; set; }
}

public class Student
{
    public int SudentId { get; set; }

    public int TeacherId { get; set; }

    public string SudentFirstName { get; set; }

    public string SudentLastName { get; set; }
}


Workaround: prevent the child templates from serializing
public class MyRadGridView : RadGridView
{
    public override string ThemeClassName
    {
        get
        {
            return typeof(RadGridView).FullName;
        }
    }

    public override void SaveLayout(string fileName)
    {
        MyGridViewLayoutSerializer ser = new MyGridViewLayoutSerializer(this.XmlSerializationInfo);
        using (XmlTextWriter writer = new XmlTextWriter(fileName, Encoding.UTF8))
        {
            writer.Formatting = Formatting.Indented;
            writer.WriteStartElement("RadGridView");
            ser.WriteObjectElement(writer, this);
        }
    }
}

public class MyGridViewLayoutSerializer : GridViewLayoutSerializer
{
    public MyGridViewLayoutSerializer(ComponentXmlSerializationInfo componentSerializationInfo)
        : base(componentSerializationInfo)
    { }

    protected override bool ShouldSerializeValue(object component, PropertyDescriptor property, PropertySerializationMetadata overwriteMetadata)
    {
        if (property.Name == "Templates")
        {
            return false;
        }

        return base.ShouldSerializeValue(component, property, overwriteMetadata);
    }
}

To reproduce:

   Sub New()
         
        InitializeComponent()
        Dim dt As New DataTable
        dt.Columns.Add("Id", GetType(Integer))
        dt.Columns.Add("Name", GetType(String))
        For index = 1 To 1000
            dt.Rows.Add(index, "Item" & index)
        Next
        Me.RadGridView1.DataSource = dt


    End Sub
    Private Sub RadButton1_Click(sender As Object, e As EventArgs) Handles RadButton1.Click
        Dim pdfFile As String = "..\..\exportedFile.pdf"
        Dim pdfExporter As New GridViewPdfExport(Me.RadGridView1)
        pdfExporter.ShowHeaderAndFooter = True
        pdfExporter.FitToPageWidth = True
        AddHandler pdfExporter.PdfExported, AddressOf pdfExporter_PdfExported
        Dim renderer As New PdfExportRenderer()
        pdfExporter.RunExport(pdfFile, renderer)

        Process.Start(pdfFile)
    End Sub

    Private Sub pdfExporter_PdfExported(sender As Object, e As System.EventArgs)

        Dim pdfFile As String = "..\..\exportedFile.pdf"
        Dim document As RadFixedDocument
        Dim provider As Telerik.Windows.Documents.Fixed.FormatProviders.Pdf.PdfFormatProvider = New Telerik.Windows.Documents.Fixed.FormatProviders.Pdf.PdfFormatProvider()
        Using stream As Stream = File.OpenRead(pdfFile)
            document = provider.Import(stream)  '<==== Error Found
            For Each page As RadFixedPage In document.Pages
                Dim editor As FixedContentEditor = New FixedContentEditor(page)
                editor.Position.Translate(page.Size.Width / 2, page.Size.Height - 50)

                Dim pageNum As Integer = document.Pages.IndexOf(page) + 1
                editor.DrawText(pageNum + " of " + document.Pages.Count)
            Next
        End Using

        Using output As Stream = File.OpenWrite(pdfFile)
            provider.Export(document, output)    '<==== Error Found
        End Using

        Process.Start(pdfFile)
    End Sub

Workaround: use the PdfExportRenderer.PdfExporting event where you have access to the document and you can make any customizations to it:
        Dim pdfFile As String = "..\..\exportedFile.pdf"
        Dim pdfExporter As New GridViewPdfExport(Me.RadGridView1)
        pdfExporter.ShowHeaderAndFooter = True
        pdfExporter.FitToPageWidth = True
        Dim renderer As New PdfExportRenderer()
        AddHandler renderer.PdfExporting, AddressOf PdfExporting
        pdfExporter.RunExport(pdfFile, renderer)

    Private Sub PdfExporting(sender As Object, e As PdfExportingEventArgs)
        Dim document As RadFixedDocument = e.Document
        Dim provider As Telerik.Windows.Documents.Fixed.FormatProviders.Pdf.PdfFormatProvider = New Telerik.Windows.Documents.Fixed.FormatProviders.Pdf.PdfFormatProvider()

        For Each page As RadFixedPage In document.Pages
            Dim editor As FixedContentEditor = New FixedContentEditor(page)
            editor.Position.Translate(page.Size.Width / 2, page.Size.Height - 50)

            Dim pageNum As Integer = document.Pages.IndexOf(page) + 1
            editor.DrawText(pageNum & " of " & document.Pages.Count)
        Next 
    End Sub

I have a GridView (Winforms v. 2017.3.1017.40, same problem with 2017.2.502.40) configured as on the attached screenshot (made some groups, made a search, and used the "excel-like" filter on a column to uncheck some values).

Then I try to click on some column headers to sort the grid.
After a few clicks, I get this error :
InvalidOperationException: La collection a été modifiée ; l'opération d'énumération peut ne pas s'exécuter.

I got different stacktrace with the same error :
InvalidOperationException: La collection a été modifiée ; l'opération d'énumération peut ne pas s'exécuter.
   at System.ThrowHelper.ThrowInvalidOperationException(ExceptionResource resource)
   at System.Collections.Generic.List`1.Enumerator.MoveNextRare()
   at System.Collections.Generic.List`1.Enumerator.MoveNext()
   at Telerik.WinControls.UI.DataGroup.<GetEnumerator>d__0.MoveNext()
   at Telerik.WinControls.UI.GridViewGroupRowInfo.get_ChildRows()
   at Telerik.WinControls.UI.GridViewRowInfo.HasChildRows()
   at Telerik.WinControls.UI.PrintGridTraverser.CanStepInHierarchy()
   at Telerik.WinControls.UI.GridTraverser.StepInHierarchy()
   at Telerik.WinControls.UI.GridTraverser.MoveNextCore()
   at Telerik.WinControls.UI.GridTraverser.MoveNext()
   at Telerik.WinControls.UI.GridViewSearchRowInfo.worker_DoWork(Object sender, DoWorkEventArgs e)
   at System.ComponentModel.BackgroundWorker.OnDoWork(DoWorkEventArgs e)
   at System.ComponentModel.BackgroundWorker.WorkerThreadStart(Object argument)

-----

InvalidOperationException: Failed to compare two elements in the array. ---> NullReferenceException: Object reference not set to an instance of an object.
   at Telerik.WinControls.UI.GridViewRowInfoComparer.CompareRows(GridViewRowInfo x, GridViewRowInfo y, SortDescriptorCollection context)
   at System.Collections.Generic.ArraySortHelper`1.InternalBinarySearch(T[] array, Int32 index, Int32 length, T value, IComparer`1 comparer)
   at System.Collections.Generic.ArraySortHelper`1.BinarySearch(T[] array, Int32 index, Int32 length, T value, IComparer`1 comparer)


   --- End of inner exception stack trace ---
   at System.Collections.Generic.ArraySortHelper`1.BinarySearch(T[] array, Int32 index, Int32 length, T value, IComparer`1 comparer)
   at System.Collections.Generic.List`1.BinarySearch(Int32 index, Int32 count, T item, IComparer`1 comparer)
   at Telerik.Collections.Generic.HybridIndex`1.PerformWithQuickSort()
   at Telerik.Collections.Generic.HybridIndex`1.get_Items()
   at Telerik.Collections.Generic.Index`1.GetEnumerator()
   at Telerik.WinControls.Data.GroupBuilder`1.Perform(IReadOnlyCollection`1 items, Int32 level, Group`1 parent)
   at Telerik.WinControls.Data.GroupBuilder`1.get_Groups()
   at Telerik.WinControls.UI.GridViewInfo.Refresh()
   at Telerik.WinControls.UI.GridViewInfo.get_ChildRows()
   at Telerik.WinControls.UI.ViewInfoTraverser.SetCollectionForStage(Boolean initializeCollection)
   at Telerik.WinControls.UI.ViewInfoTraverser.ChangeCollectionForward()
   at Telerik.WinControls.UI.ViewInfoTraverser.MoveNextCore()
   at Telerik.WinControls.UI.ViewInfoTraverser.MoveNext()
   at Telerik.WinControls.UI.GridTraverser.MoveNextCore()
   at Telerik.WinControls.UI.GridTraverser.MoveNext()
   at Telerik.WinControls.UI.BaseGridNavigator.NavigateToRow(GridViewRowInfo row, Boolean select)
   at Telerik.WinControls.UI.BaseGridNavigator.ProcessViewChangedEvent(GridViewEvent eventData)
   at Telerik.WinControls.UI.BaseGridNavigator.ProcessEventCore(GridViewEvent eventData)
   at Telerik.WinControls.UI.GridViewEventProcessEntity.ProcessCollection(GridViewEvent gridEvent, PriorityWeakReferenceList list, GridEventProcessMode processMode)
   at Telerik.WinControls.UI.GridViewEventProcessEntity.ProcessEvent(GridViewEvent gridEvent)
   at Telerik.WinControls.UI.GridViewSynchronizationService.NotifyListeners(GridViewEvent gridEvent)
   at Telerik.WinControls.UI.GridViewSynchronizationService.FlushEvents()
   at Telerik.WinControls.UI.GridViewSynchronizationService.DispatchEvent(GridViewEvent gridEvent)
   at Telerik.WinControls.UI.GridViewSynchronizationService.DispatchEvent(GridViewTemplate template, GridViewEvent eventData, Boolean postUI)
   at Telerik.WinControls.UI.GridViewTemplate.DispatchDataViewChangedEvent(Object sender, DataViewChangedEventArgs args)
   at Telerik.WinControls.UI.GridViewTemplate.OnViewChanged(Object sender, DataViewChangedEventArgs e)
   at Telerik.WinControls.UI.MasterGridViewTemplate.OnViewChanged(Object sender, DataViewChangedEventArgs e)
   at Telerik.WinControls.Data.RadDataView`1.RebuildData(Boolean notify)
   at Telerik.WinControls.Data.RadDataView`1.OnNotifyPropertyChanged(PropertyChangedEventArgs e)
   at Telerik.WinControls.Data.RadCollectionView`1.OnNotifyPropertyChanged(String propertyName)
   at Telerik.WinControls.UI.GridViewSortDescriptorCollection.OnCollectionChanged(NotifyCollectionChangedEventArgs args)
   at System.ComponentModel.PropertyChangedEventHandler.Invoke(Object sender, PropertyChangedEventArgs e)
   at Telerik.WinControls.Data.SortDescriptor.set_Direction(ListSortDirection value)
   at Telerik.WinControls.UI.GridViewColumn.Sort(RadSortOrder sortOrder, Boolean multiSortMode)
   at Telerik.WinControls.UI.GridHeaderCellElement.Sort(RadSortOrder sortOrder)
   at Telerik.WinControls.UI.GridHeaderRowBehavior.OnMouseUp(MouseEventArgs e)
   at Telerik.WinControls.UI.BaseGridBehavior.OnMouseUp(MouseEventArgs e)
   at Telerik.WinControls.UI.RadGridView.OnMouseUp(MouseEventArgs e)
   at System.Windows.Forms.Control.WmMouseUp(Message& m, MouseButtons button, Int32 clicks)
   at System.Windows.Forms.Control.WndProc(Message& m)
   at Telerik.WinControls.RadControl.WndProc(Message& m)
   at Telerik.WinControls.UI.RadGridView.WndProc(Message& m)
   at System.Windows.Forms.NativeWindow.Callback(IntPtr hWnd, Int32 msg, IntPtr wparam, IntPtr lparam)

------

InvalidOperationException: Collection was modified; enumeration operation may not execute.
   at System.ThrowHelper.ThrowInvalidOperationException(ExceptionResource resource)
   at System.Collections.Generic.List`1.Enumerator.MoveNextRare()
   at System.Collections.Generic.List`1.Enumerator.MoveNext()
   at Telerik.WinControls.UI.DataGroup.<GetEnumerator>d__0.MoveNext()
[...]
   at System.Collections.Generic.List`1..ctor(IEnumerable`1 collection)
   at System.Linq.Enumerable.ToList[TSource](IEnumerable`1 source)
   at Alto.Client.Controls.Dispatch.DispatchResources.DeliveryPointFilterManager.get_DeliveryPointsVisible() in d:\Builds\45153\Alto\Alto.Prod\src\Alto\Alto.Client\Controls\Dispatch\DispatchResources\DeliveryPointFilterManager.cs:line 38
   at Alto.Client.Controls.Dispatch.DispatchResources.UcDeliveryPoint.get_Title() in d:\Builds\45153\Alto\Alto.Prod\src\Alto\Alto.Client\Controls\Dispatch\DispatchResources\UcDeliveryPoint.cs:line 1520
   at Alto.Client.Controls.Dispatch.DispatchResources.UcDeliveryPoint.updatePanelTitle() in d:\Builds\45153\Alto\Alto.Prod\src\Alto\Alto.Client\Controls\Dispatch\DispatchResources\UcDeliveryPoint.cs:line 543
   at Alto.Client.Controls.Dispatch.DispatchResources.UcDeliveryPoint.onFilterChanged(Object sender, GridViewCollectionChangedEventArgs e) in d:\Builds\45153\Alto\Alto.Prod\src\Alto\Alto.Client\Controls\Dispatch\DispatchResources\UcDeliveryPoint.cs:line 547
   at Telerik.WinControls.UI.GridViewCollectionChangedEventHandler.Invoke(Object sender, GridViewCollectionChangedEventArgs e)
   at Telerik.WinControls.UI.RadGridView.OnFilterChanged(Object sender, GridViewCollectionChangedEventArgs e)
   at Telerik.WinControls.UI.EventDispatcher.RaiseEvent[T](Object eventKey, Object sender, T args)
   at Telerik.WinControls.UI.GridViewFilterDescriptorCollection.OnCollectionChanged(NotifyCollectionChangedEventArgs args)
   at Telerik.Collections.Generic.NotifyCollection`1.InsertItem(Int32 index, T item)
   at Telerik.WinControls.UI.GridViewFilterDescriptorCollection.InsertItem(Int32 index, FilterDescriptor item)
   at System.Collections.ObjectModel.Collection`1.Add(T item)
   at Telerik.WinControls.UI.GridViewDataColumn.SetFilterDescriptor(FilterDescriptor value)
   at Telerik.WinControls.UI.GridHeaderCellElement.filterPopup_FilterConfirmed(Object sender, EventArgs e)
   at Telerik.WinControls.UI.BaseFilterPopup.OnFilterConfirmed()
   at Telerik.WinControls.UI.RadListFilterPopup.OnButtonOkClick(EventArgs e)
   at Telerik.WinControls.UI.RadListFilterPopup.ButtonOK_Click(Object sender, EventArgs e)
   at Telerik.WinControls.RadElement.OnClick(EventArgs e)
   at Telerik.WinControls.UI.RadButtonItem.OnClick(EventArgs e)
   at Telerik.WinControls.UI.RadButtonElement.OnClick(EventArgs e)
   at Telerik.WinControls.RadElement.DoClick(EventArgs e)
   at Telerik.WinControls.RadElement.RaiseBubbleEvent(RadElement sender, RoutedEventArgs args)
   at Telerik.WinControls.RadItem.RaiseBubbleEvent(RadElement sender, RoutedEventArgs args)
   at Telerik.WinControls.RadElement.RaiseRoutedEvent(RadElement sender, RoutedEventArgs args)
   at Telerik.WinControls.RadElement.RaiseBubbleEvent(RadElement sender, RoutedEventArgs args)
   at Telerik.WinControls.RadItem.RaiseBubbleEvent(RadElement sender, RoutedEventArgs args)
   at Telerik.WinControls.RadElement.RaiseRoutedEvent(RadElement sender, RoutedEventArgs args)
   at Telerik.WinControls.RadElement.DoMouseUp(MouseEventArgs e)
   at Telerik.WinControls.ComponentInputBehavior.OnMouseUp(MouseEventArgs e)
   at Telerik.WinControls.RadControl.OnMouseUp(MouseEventArgs e)
   at System.Windows.Forms.Control.WmMouseUp(Message& m, MouseButtons button, Int32 clicks)
   at System.Windows.Forms.Control.WndProc(Message& m)
   at System.Windows.Forms.ScrollableControl.WndProc(Message& m)
   at Telerik.WinControls.RadControl.WndProc(Message& m)
   at Telerik.WinControls.UI.RadPopupControlBase.WndProc(Message& m)
   at System.Windows.Forms.Control.ControlNativeWindow.OnMessage(Message& m)
   at System.Windows.Forms.Control.ControlNativeWindow.WndProc(Message& m)
   at System.Windows.Forms.NativeWindow.Callback(IntPtr hWnd, Int32 msg, IntPtr wparam, IntPtr lparam)

----

InvalidOperationException: Collection was modified; enumeration operation may not execute.
   at System.Collections.Generic.List`1.Enumerator.MoveNextRare()
   at Telerik.WinControls.UI.DataGroup.<GetEnumerator>d__0.MoveNext()
   at Telerik.WinControls.Data.GroupBuilder`1.Perform(IReadOnlyCollection`1 items, Int32 level, Group`1 parent)
   at Telerik.WinControls.Data.DataItemGroup`1.get_Groups()
   at Telerik.WinControls.UI.DataGroup.get_Groups()
   at Telerik.WinControls.UI.GridViewTemplate.InvalidateSummaryRowsCore(DataGroup group, Boolean invalidateChildren)
   at Telerik.WinControls.UI.GridViewTemplate.InvalidateSummaryRowsCore(DataGroup group, Boolean invalidateChildren)
   at Telerik.WinControls.UI.GridViewTemplate.InvalidateGroupSummaryRows(DataGroup group, Boolean invalidateChildren)
   at Telerik.WinControls.UI.GridViewTemplate.RefreshSummaryRowsInGroup(NotifyCollectionChangedEventArgs e)
   at Telerik.WinControls.UI.GridViewTemplate.RefreshAggregates(NotifyCollectionChangedEventArgs e)
   at Telerik.WinControls.UI.MasterGridViewTemplate.OnViewChanged(Object sender, DataViewChangedEventArgs e)
   at Telerik.WinControls.Data.RadDataView`1.RebuildData(Boolean notify)
   at Telerik.WinControls.Data.RadDataView`1.OnNotifyPropertyChanged(PropertyChangedEventArgs e)
   at Telerik.WinControls.Data.RadCollectionView`1.OnNotifyPropertyChanged(String propertyName)
   at Telerik.WinControls.UI.GridViewSortDescriptorCollection.OnCollectionChanged(NotifyCollectionChangedEventArgs args)
   at Telerik.WinControls.UI.GridViewSortDescriptorCollection.RemoveItem(Int32 index)
   at Telerik.WinControls.UI.GridViewColumn.Sort(RadSortOrder sortOrder, Boolean multiSortMode)
   at Telerik.WinControls.UI.GridHeaderCellElement.Sort(RadSortOrder sortOrder)
   at Telerik.WinControls.UI.GridHeaderRowBehavior.OnMouseUp(MouseEventArgs e)
   at Telerik.WinControls.UI.BaseGridBehavior.OnMouseUp(MouseEventArgs e)
   at Telerik.WinControls.UI.RadGridView.OnMouseUp(MouseEventArgs e)
   at System.Windows.Forms.Control.WmMouseUp(Message& m, MouseButtons button, Int32 clicks)
   at System.Windows.Forms.Control.WndProc(Message& m)
   at Telerik.WinControls.RadControl.WndProc(Message& m)
   at Telerik.WinControls.UI.RadGridView.WndProc(Message& m)
   at System.Windows.Forms.NativeWindow.Callback(IntPtr hWnd, Int32 msg, IntPtr wparam, IntPtr lparam)

The new mode should allow copying of single cells even when having the SelectionMode set to FullRowSelect. The attached project features a possible workaround.

To reproduce:

            this.radGridView1.Columns.Add("FIRST CELL ID A");

            this.radGridView1.Rows.Add("405-55-214-41763");
            this.radGridView1.Rows.Add("405-55-214-46682");
            this.radGridView1.Rows.Add("405-55-214-46682");

            this.radGridView1.AutoSizeColumnsMode = Telerik.WinControls.UI.GridViewAutoSizeColumnsMode.Fill;
            this.radGridView1.MultiSelect = true;
            this.radGridView1.SelectionMode = Telerik.WinControls.UI.GridViewSelectionMode.CellSelect;

Please refer to the attached gif file illustrating better the text moving. 

Workaround: 
        private void radGridView1_CellFormatting(object sender, Telerik.WinControls.UI.CellFormattingEventArgs e)
        {
            e.CellElement.BorderBottomWidth = 2;
            e.CellElement.BorderRightWidth = 2;
            e.CellElement.BorderTopWidth = 2;
            e.CellElement.BorderLeftWidth = 2;
        }

Please refer to the attached sample project and follow the steps illustrated in the gif file.

Note: the same behavior is observed with the CellValidating event in the new row.

Workaround: use the RowValidating event instead:

        private void radGridView1_RowValidating(object sender, Telerik.WinControls.UI.RowValidatingEventArgs e)
        {
            if (e.Row is Telerik.WinControls.UI.GridViewNewRowInfo && e.Row.Cells["Id"].Value == null)
            {
                this.radGridView1.MasterView.TableAddNewRow.CancelAddNewRow();
                this.radGridView1.CurrentRow = this.radGridView1.MasterView.TableAddNewRow;
            }
        }
 
NOTE: The CellValidating event is fired multiple times as well when you select a new tab in RadDock. There is a second project attached.

To reproduce: if you pin a calculator or a hyperlink column you will notice that the cells belonging to these columns remain white. However, the cells from other column types  have a light blue/gray fill color for pinned state. 

Workaround: 
private void RadGridView1_CellFormatting(object sender, CellFormattingEventArgs e)
{
    if (e.Column.IsPinned)
    {
        e.CellElement.BackColor = Color.FromArgb(235, 244, 252);
    }
    else
    {
        e.CellElement.ResetValue(LightVisualElement.BackColorProperty, ValueResetFlags.Local);
    }
}

To reproduce:

Please run the sample project and follow the steps from the gif file.

Workaround:

        private void radGridView1_SizeChanged(object sender, EventArgs e)
        {
            this.radGridView1.MasterTemplate.Refresh();
        }

        public class Grid : RadGridView
        {
            public override string ThemeClassName  
            { 
                get 
                { 
                    return typeof(RadGridView).FullName;  
                }
            }

            protected override void OnResize(EventArgs e)
            {
                object value = null;
                if (this.ActiveEditor != null)
                { 
                    value = this.ActiveEditor.Value;  
                }
                base.OnResize(e);
                if (value != null && this.CurrentCell != null)
                {
                    this.CurrentCell.Value = value;
                }
            }
        }

To reproduce: run the attached sample project. 

Type "chef" in the search box and then try to sort some of the columns. Click the header cell several times and as a result the exception will occur.

Workaround:

        public class CustomGrid : RadGridView
        {
            public new object DataSource
            { 
                get
                {
                    return (object)base.DataSource;
                }
                set
                {
                    GridViewSearchRowInfo.Cancel = true;
                    Thread.Sleep(100);
                    base.DataSource = null;
                    this.EnableFiltering = false;
                    base.DataSource = value;
                    this.EnableFiltering = true;
                    base.CurrentRow = null; 
                }
            }
        }

A similar issue can be also observed in RadTreeView:

Workaround RadGridView: 
private void RadGridView1_CellEditorInitialized(object sender, GridViewCellEventArgs e)
{
    RadTextBoxEditor editor = e.ActiveEditor as RadTextBoxEditor;
    if (editor != null)
    {
        editor.EditorElement.MinSize = new Size(0, 30);
    }
}

Workaround RadTreeView:
 
private void RadTreeView1_EditorInitialized(object sender, TreeNodeEditorInitializedEventArgs e)
{
    TreeViewTextBoxEditor editor = e.Editor as TreeViewTextBoxEditor;
    if (editor != null)
    {
        ((BaseTextBoxEditorElement)editor.EditorElement).TextBoxItem.TextBoxControl.MinimumSize = new Size(0, 24);
    }
}

In 125% the RadTextBox in the Fluent theme keeps shrinking:

Workaround
public RadForm1()
{
    InitializeComponent();

    SizeF dpi = NativeMethods.GetMonitorDpi(Screen.PrimaryScreen, NativeMethods.DpiType.Effective);
    this.radTextBox1.MinimumSize = new Size(0, (int)(24 * dpi.Height));
}

To reproduce: run the attached sample project. Type "res" in the search box very very fast. You will notice that the results with starting with "re" are highlighted. 

Note: it is usually reproducible the first serahc after loading the form. 

To reproduce: setup self-reference hierarchy. Add a GridViewHyperlinkColumn and show the expander item in this column. Please refer to the attached screenshot. The hyperlink text overlaps with the expander.

Workaround: set the MasterTemplate.SelfReferenceExpanderColumn to a different column.