The GridViewSelectColumn displays a CheckBox in its header, which selects all rows in the column. If the CheckBox is unchecked, and you click it to check the items, the items get selected/checked. However, the CheckBox doesn't get check until the second click.

This reproduces only when the RadGridView is grouped and also the groups are collapsed so that no data records are visible.

To work this around, you can replace the default CheckBox with a new one in the column's Header. Then, manually select and deselect the items. You can use the CheckBox MouseLeftButtonDown or Checked/Unchecked events. Note that it is important to wrap the CheckBox in the Header, in another control (like a Grid in the example below), in order to avoid the default RadGridView logic to kick-in.

  <telerik:GridViewSelectColumn>
      <telerik:GridViewSelectColumn.Header>
          <Grid>
              <CheckBox PreviewMouseLeftButtonDown="CheckBox_PreviewMouseLeftButtonDown"/>
          </Grid>
      </telerik:GridViewSelectColumn.Header>
  </telerik:GridViewSelectColumn>

 private void CheckBox_PreviewMouseLeftButtonDown(object sender, System.Windows.Input.MouseButtonEventArgs e)
 {
     var checkBox = (CheckBox)sender;            
     if (checkBox.IsChecked.Value)
     {
         this.GridView.SelectedItems.Clear();
     }
     else
     {
         this.GridView.SelectAll();
     }
 }

When the RadGridView is filtered you can get all items in the data view using the Items collection property of the control. The count can be accessed with the gridView.Items.Count property.

If the RadGridView is grouped and then filtered, the Items.Count no longer returns the correct value. The count doesn't take into account the items that are in collapsed groups. Instead the count contains only the expanded group objects.

To work this around, use the following code instead of gridView.Items.Count.

int count = gridView.Items.OfType<object>().Count(); 

The selection with the Shift key when SelectionMode=Extended no longer works in the default Nested rendering mode of RadGridView. This reproduces only when the data view is grouped.

To work this around, set the GroupRenderMode property of RadGridView to Flat.

<telerik:RadGridView GroupRenderMode="Flat" />

The cell content gets clipped when the width of the previous column changes at runtime. This happens in the Windows11 and Office2019 themes (possibly in others) and only if the clipped cell is position before the right frozen columns area.

To work this around, you can call the Rebind() method of RadGridView, after the column width changes.

Arrow keys (up and down) don't scroll the first row which is outside of the viewport. One way to reproduce this is to scroll down, then select the topmost visible row and press the Up arrow key in order to select the row above. This should scroll the view a bit up in order to see the previous row which is now selected.
Currently, this doesn't work when using a custom implementation of the GridViewCell class.
The issue is reproducible when GroupRenderMode is set to Flat.

To work this around, you can manually scroll to the row.

public MainWindow()
{
	InitializeComponent();
	this.gridView.AddHandler(RadGridView.KeyUpEvent, new KeyEventHandler(OnGridViewKeyUp), true);
}

private void OnGridViewKeyUp(object? sender, System.Windows.Input.KeyEventArgs e)
{
	if (e.Key == Key.Up || e.Key == Key.Down)
	{
		Dispatcher.BeginInvoke(new Action(() =>
		{
			var currentCell = gridView.CurrentCell;
			var row = currentCell.ParentRow;

			// Since the VisualOffset proeprty is protected and you are already inheriting the GridViewRow class, you can just expose it through an extra property in the CustomGridViewRow, instead of using reflection like in this example.
			var visualOffsetPropInfo = typeof(Visual).GetProperty("VisualOffset", System.Reflection.BindingFlags.Instance | System.Reflection.BindingFlags.NonPublic); 			
			var offset = (Vector)visualOffsetPropInfo.GetValue(row);
			if (offset.Y < 0)
			{
				row.BringIntoView();
			}
		}));
	}
}

InvalidCastException is thrown when the DropIndicatorThickness property of the GridViewHeaderCell is set. Or whenever the DropIndicatorBrush property is accessed. This happens when using the Windows 7 theme. Also, the EnableColumnVirtualization property should be set to False.

System.InvalidCastException: 'Unable to cast object of type 'System.Windows.Media.LinearGradientBrush' to type 'System.Windows.Media.SolidColorBrush'.'

To work this around, you can use an implicit style targeting the GridViewHeaderCell in order to set its DropIndicatorBrush to a SolidColorBrush value.

 <telerik:RadGridView.Resources>
     <Style TargetType="telerik:GridViewHeaderCell">
         <Setter Property="DropIndicatorBrush" Value="White" />
         <Setter Property="DropIndicatorThickness" Value="10" />
     </Style>
 </telerik:RadGridView.Resources>

Column groups may disappear when the RadGridView control is hosted in a TabItem of TabControl. These column groups can disappear when new GridViewColumnGroup instances are created.

Furthermore, resizing a column of RadGridView will result in a NullReferenceException.

To work this around, set the EnableColumnGroupsVirtualization property of RadGridView to False.