Use attached to reproduce.

Workaround:

class MyViewDefinition : TableViewDefinition
{
    public override IRowView CreateViewUIElement(GridViewInfo viewInfo)
    {
        return new MyTableElement();
    }
}
class MyTableElement : GridTableElement
{
    public override void DpiScaleChanged(SizeF scaleFactor)
    {
        if (this.ViewTemplate != null)
        {
            base.DpiScaleChanged(scaleFactor);
        }

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

//use the above definition like this:

 radGridView1.ViewDefinition = new MyViewDefinition();

To reproduce: run the attached sample project:

1. Clear the value in one of the cells.
2. Select the empty cell and press Ctrl+C to copy the empty cell. You will encounter the error coming from the GetFormattedCellValue method.

Workaround: handle the RadGridView.Copying event

private void RadGridView1_Copying(object sender, GridViewClipboardEventArgs e)
{
    if (this.radGridView1.SelectionMode == GridViewSelectionMode.FullRowSelect)
    {
        foreach (var row in this.radGridView1.SelectedRows)
        {
            foreach (var cell in row.Cells)
            {
                GridViewCellInfo cellInfo = cell as GridViewCellInfo;
                if (cellInfo != null && cellInfo.Value == null)
                {
                    cellInfo.Value = "";
                }
            }
        }
    }
    else
    {
        foreach (var cell in this.radGridView1.SelectedCells)
        {
            if (cell.Value == null)
            {
                cell.Value = "";
            }
        }
    }
}

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);
 }

We'd like to see the ability to enable a search/filter function for column chooser.  One of our applications that uses the RadGridView has dozens of columns, most hidden by default.  Our users would like the ability to type in part of a column name and have the column chooser filter on it.

To reproduce:
1. Filter the first column in the grid
2. Click the header checkbox
3. Try to clear the filter. you iwll notice that the data is still filtered.

Workaround:

private void radGridView1_CreateCell(object sender, GridViewCreateCellEventArgs e)
{
    if (e.CellType==typeof(GridCheckBoxHeaderCellElement))
    {
        e.CellElement = new CustomHeaderCell(e.Column, e.Row);
    }
}

public class CustomHeaderCell : GridCheckBoxHeaderCellElement
{

    public CustomHeaderCell(GridViewColumn column, GridRowElement row) : base(column, row)
    {
    }

    protected override void checkbox_ToggleStateChanged(object sender, StateChangedEventArgs args)
    {
        base.checkbox_ToggleStateChanged(sender, args);
        this.TableElement.EndUpdate(false);
    }
}

How to reproduce: Check the attached video --> radgridview-filtering.gif

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

        this.radGridView1.DataSource = this.GetData();
        this.radGridView1.AutoSizeColumnsMode = GridViewAutoSizeColumnsMode.Fill;

        this.radGridView1.EnableFiltering = true;
        this.radGridView1.FilterChanging += RadGridView1_FilterChanging;
        this.radGridView1.FilterChanged += RadGridView1_FilterChanged;
    }

    private void RadGridView1_FilterChanged(object sender, GridViewCollectionChangedEventArgs e)
    {
        if (clearFiler)
        {
            this.radGridView1.FilterDescriptors.Remove((FilterDescriptor)e.NewItems[0]);
            clearFiler = false;
        }
    }

    bool clearFiler = false;
    private void RadGridView1_FilterChanging(object sender, GridViewCollectionChangingEventArgs e)
    {
        if (e.PropertyName == "Operator" && e.Action == Telerik.WinControls.Data.NotifyCollectionChangedAction.ItemChanging)
        {
            FilterDescriptor fd = e.OldItems[0] as FilterDescriptor;
            if (fd != null && (fd.Operator == FilterOperator.IsNull || fd.Operator == FilterOperator.IsNotNull))
            {
                clearFiler = true;
            }
        }
    }
    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));
        dt.Columns.Add("Bool", typeof(bool));

        for (int i = 0; i < 100; i++)
        {
            dt.Rows.Add(i, "Name " + i, DateTime.Now.AddDays(i), i % 2 == 0);
        }

        return dt;
    }
}

Workaround:
Use the Copying event.

The CustomSorting event should manipulate the pinned rows as well. Thus, the user will be allowed to control the sort order of the pinned rows within the pinned container.

Steps to reproduce: 
1. Set DPI scale to 150 of all screens 
2. Run attached sample 
3. The column`s width is not calculated correctly. 

Attached project for reproducing.

Workaround:
var column = radGridView1.Columns[2] as GridViewDecimalColumn;
var format = column.FormatString;
column.FormatString = "";
column.ExcelExportType = DisplayFormatType.Currency;

//export the grid

column.FormatString = format;

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

To reproduce:
        public Form1()
        {
            InitializeComponent();
            DataTable dt = new DataTable();
            dt.Columns.Add("Id", typeof(int));
            dt.Columns.Add("Name", typeof(string));
            dt.Columns.Add("ParentId", typeof(int));

            for (int i = 0; i < 3; i++)
            {
                dt.Rows.Add(i, "Parent" + i, -1);
            }
            Random rand = new Random();
            for (int i = 3; i < 15; i++)
            {
                dt.Rows.Add(i,"Child"+i,rand.Next(0,3));
            }

            this.radGridView1.Relations.AddSelfReference(this.radGridView1.MasterTemplate, "Id", "ParentId");
            this.radGridView1.DataSource = dt;
            this.radGridView1.AutoSizeColumnsMode = GridViewAutoSizeColumnsMode.Fill;
            this.radGridView1.EnableGrouping = true;
            this.radGridView1.EnableCustomGrouping = true;
            this.radGridView1.CustomGrouping+=radGridView1_CustomGrouping;
         }

private void radGridView1_CustomGrouping(object sender, GridViewCustomGroupingEventArgs e)
{
     
}

To reproduce:
private void RadGridView1_KeyPress(object sender, KeyPressEventArgs e)
{
    int startIndex = radGridView1.CurrentRow.Index +1;

    for (int i = startIndex; i < radGridView1.Rows.Count; i++)
    {
        var value = radGridView1.Rows[i].Cells["Name"].Value.ToString().ToLower();
        if (value.StartsWith(e.KeyChar.ToString()))
        {
            radGridView1.TableElement.ScrollToRow(radGridView1.Rows[i]);
            radGridView1.ClearSelection();
            radGridView1.Rows[i].IsSelected = true;
            radGridView1.Rows[i].IsCurrent = true;
            radGridView1.Columns[1].IsCurrent = true;
            break;
        }
    }
}

- Press some key so the grid is scrolled down, then press the down key.
- The first row is selected instead of the next one. 

Workaround:
remove this line:
radGridView1.Rows[i].IsSelected = true;

To reproduce:
1. Run the attached sample project.
2. Right-click the grid.
3. Quickly navigate to another application (e.g. Windows Explorer) before the context menu is shown & wait at windows explorer to display the menu
4. The context will be stuck.

To reproduce: please refer to the attached sample project and follow the steps in the provided gif file: 
1. Enter something in the search bar
2. Filter on some column, uncheck "All", then select a few items, then validate
3. Click on a random row on the grid
4. Filter on the same column as in 2., check "All", then uncheck a few items, then validate

Workaround:

private void radGridView1_FilterChanged(object sender, Telerik.WinControls.UI.GridViewCollectionChangedEventArgs e)
{
    this.radGridView1.MasterTemplate.Refresh();
}

Please refer to the attached sample project and gif file illustrating the behavior.
ColumnGroupsViewDefinition view = this.radGridView1.ViewDefinition as ColumnGroupsViewDefinition;

            foreach (GridViewColumnGroup group in view.ColumnGroups)
            {
                group.ShowHeader = false;
            }

Workaround: use the ViewCellFormatting and enable the back color for the header cells that are over the hidden group headers

public RadForm1()
{
    InitializeComponent();

    ColumnGroupsViewDefinition view = this.radGridView1.ViewDefinition as ColumnGroupsViewDefinition ;
    foreach (GridViewColumnGroup group in view.ColumnGroups)
    {
        group.ShowHeader = false;
    }
}

private void radGridView1_ViewCellFormatting(object sender, CellFormattingEventArgs e)
{
    if (e.Row is GridViewTableHeaderRowInfo && !(e.CellElement is GridColumnGroupCellElement))
    {
        e.CellElement.DrawFill = true;
        e.CellElement.BackColor = Color.FromArgb(234, 242, 252);
        e.CellElement.GradientStyle = GradientStyles.Solid;
    }
    else
    { 
        e.CellElement.ResetValue(LightVisualElement.DrawFillProperty, ValueResetFlags.Local);
        e.CellElement.ResetValue(LightVisualElement.BackColorProperty, ValueResetFlags.Local);
        e.CellElement.ResetValue(LightVisualElement.GradientStyleProperty, ValueResetFlags.Local);
    }
}

How to reproduce:
public partial class Form1 : Form
{
    public class Data
    {
        public bool Checked { get; set; }
        public string Text { get; set; }
    }

    private readonly RadGridView _grid;

    private readonly List<Data> _dataList = new List<Data>
    {
        new Data {Text = "abc"},
        new Data {Text = "def"},
        new Data {Text = "ghi"},
        new Data {Text = "jkl"},
        new Data {Text = "mno"},
        new Data {Text = "pqr"},
        new Data {Text = "stu"},
        new Data {Text = "vwx"},
        new Data {Text = "yz0"}
    };

    public Form1()
    {
        InitializeComponent();

        _grid = new RadGridView
        {
            Dock = DockStyle.Fill,
            AllowSearchRow = true
        };

        Controls.Add(_grid);

        _grid.DataSource = _dataList;
    }

    protected override void OnLoad(EventArgs e)
    {
        base.OnLoad(e);

        _grid.AutoSizeColumnsMode = GridViewAutoSizeColumnsMode.Fill;
        ((GridViewCheckBoxColumn)_grid.Columns["Checked"]).EnableHeaderCheckBox = true;
        
        _grid.Columns["Checked"].SortOrder = RadSortOrder.Ascending;
        ((GridViewCheckBoxColumn)_grid.Columns["Checked"]).Checked = ToggleState.On;
        _grid.MasterView.TableSearchRow.Search("abc");
    }
}

Workaround: clear the search text
protected override void OnLoad(EventArgs e)
{
    base.OnLoad(e);

    _grid.AutoSizeColumnsMode = GridViewAutoSizeColumnsMode.Fill;
    ((GridViewCheckBoxColumn)_grid.Columns["Checked"]).EnableHeaderCheckBox = true;
    
    _grid.Columns["Checked"].SortOrder = RadSortOrder.Ascending;
    ((GridViewCheckBoxColumn)_grid.Columns["Checked"]).Checked = ToggleState.On;
    _grid.MasterView.TableSearchRow.Search("abc");

    this._grid.MouseDown += _grid_MouseDown;
}

private void _grid_MouseDown(object sender, MouseEventArgs e)
{
    RadGridView grid = (RadGridView)sender;
    RadCheckBoxElement cell = grid.ElementTree.GetElementAtPoint(e.Location) as RadCheckBoxElement;
    if (cell != null && cell.Parent is GridCheckBoxHeaderCellElement && grid.SortDescriptors.Count > 0))
    {
        grid.MasterView.TableSearchRow.Search("");
    }
}

To reproduce:
protected override void OnLoad(EventArgs e)
{
    base.OnLoad(e);
    DataTable dt = NewDt();
    this.radGridView1.DataSource = dt;      
    var c = this.radGridView1.Columns["Calculted"] as GridViewDecimalColumn;
    c.EnableExpressionEditor = true;
    c.Expression = "Column2 / SUM(Column2) * 10";
    c.DecimalPlaces = 2;
    c.FormatString = "{0:N2}";

    this.radGridView1.GroupDescriptors.Add("GroupByColumn", ListSortDirection.Ascending);
}

private DataTable NewDt()
{
    var dt = new DataTable();
    dt.Columns.Add("ItemColumn");
    dt.Columns.Add("GroupByColumn");
    dt.Columns.Add("Column2", typeof(decimal));
    dt.Columns.Add("Calculted", typeof(decimal));

    for (int i = 1; i <= 20; i++)
    {
        string gr = string.Empty;
        if (i % 3 == 0)
            gr = "gr3";
        else if (i % 2 == 0)
            gr = "gr2";
        else if (i % 5 == 0)
            gr = "gr5";
        else
            gr = "item" + i.ToString();

        dt.Rows.Add("id #" + i.ToString(), gr, i, 0);
    }
    return dt;
}

Workaround:
public class CustomExpressionContext : Telerik.Data.Expressions.ExpressionContext
{

    RadGridView radGridView;
    public CustomExpressionContext(RadGridView grid)
    {
        this.radGridView = grid;
    }

    public decimal MySum(string columnName)
    {
        decimal sum = 0;
        
        foreach (var item in radGridView.Rows)
        {
            sum += (decimal)item.Cells[columnName].Value;
        }
        return sum;
    }
}

 Telerik.Data.Expressions.ExpressionContext.Context = new CustomExpressionContext(this.radGridView1);