To reproduce: use the following code snippet and follow the steps:
public RadForm1()
{
    InitializeComponent();

    this.radVirtualGrid1.RowCount = 10;
    this.radVirtualGrid1.ColumnCount = 5;
    this.radVirtualGrid1.CellValueNeeded+=radVirtualGrid1_CellValueNeeded;
}

private void radVirtualGrid1_CellValueNeeded(object sender, Telerik.WinControls.UI.VirtualGridCellValueNeededEventArgs e)
{
    e.Value = e.RowIndex + "." + e.ColumnIndex;
}

1. Pin (left or right) 1st column
2. Scroll horizontally (to not display 2nd column)
3. Unpin 1st column
4. Scroll horizontally to display 1st column
=> 1st column is hidden, the only way to make it visible is to resize a column or the entire window

Workaround: 

 private void radVirtualGrid1_ContextMenuOpening(object sender, Telerik.WinControls.UI.VirtualGridContextMenuOpeningEventArgs e)
 {
     foreach (RadItem item in e.ContextMenu.Items)
     {
         if (item.Text == 
             RadVirtualGridLocalizationProvider.CurrentProvider.GetLocalizedString(RadVirtualGridStringId.PinMenuItem))
         {
             foreach (RadMenuItem subItem in ((RadMenuItem)item).Items)
             {
                 if (subItem.Text == 
                     RadVirtualGridLocalizationProvider.CurrentProvider.GetLocalizedString(RadVirtualGridStringId.UnpinColumnMenuItem))
                 {
                     subItem.Click-=subItem_Click;
                     subItem.Click+=subItem_Click;
                 }
             }
         }
     }
 }

 private void subItem_Click(object sender, EventArgs e)
 {
      this.radVirtualGrid1.MasterViewInfo.SetColumnWidth(0, this.radVirtualGrid1.MasterViewInfo.GetColumnWidth(0));
 }

To reproduce: use the following code snippet and try to select another cell.

public RadForm1()
{
    InitializeComponent();

    this.radVirtualGrid1.RowCount = 30;
    this.radVirtualGrid1.ColumnCount = 5;
    this.radVirtualGrid1.CellValueNeeded += radVirtualGrid1_CellValueNeeded;
      
    this.radVirtualGrid1.CurrentCellChanging += radVirtualGrid1_CurrentCellChanging;
}

private void radVirtualGrid1_CurrentCellChanging(object sender, Telerik.WinControls.UI.VirtualGridCellInfoCancelEventArgs e)
{
    e.Cancel = true;
}

private void radVirtualGrid1_CellValueNeeded(object sender, Telerik.WinControls.UI.VirtualGridCellValueNeededEventArgs e)
{
    e.Value = e.RowIndex + "." + e.ColumnIndex;
}

Workaround:

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

    protected override void OnMouseDown(MouseEventArgs e)
    {
        if (e.Button == System.Windows.Forms.MouseButtons.Left)
        {
            return;
        }
        base.OnMouseDown(e);
    }
}

To reproduce:
private void RadVirtualGrid1_EditorRequired(object sender, Telerik.WinControls.UI.VirtualGridEditorRequiredEventArgs e)
{
    switch (e.ColumnIndex)
    {
        case 1:
            {
                VirtualGridTextBoxControlEditor editor = new VirtualGridTextBoxControlEditor();
                editor.CharacterCasing = CharacterCasing.Upper;
                e.Editor = editor;
            }
            break;
    }
}

- Open the editor and try using the backspace or the arrow keys. 

Workaround:
class MyGrid : RadVirtualGrid
{
    protected override void OnKeyDown(KeyEventArgs e)
    {
        base.OnKeyDown(e);

        if (this.ActiveEditor is VirtualGridTextBoxControlEditor)
        {
            var editor = this.ActiveEditor as VirtualGridTextBoxControlEditor;
            var element =   editor.EditorElement as RadTextBoxControlElement;
            element.InputHandler.ProcessKeyDown(e);
        }
    }
    protected override void OnKeyUp(KeyEventArgs e)
    {
        base.OnKeyUp(e);

        if (this.ActiveEditor is VirtualGridTextBoxControlEditor)
        {
            var editor = this.ActiveEditor as VirtualGridTextBoxControlEditor;
            var element = editor.EditorElement as RadTextBoxControlElement;
            element.InputHandler.ProcessKeyUp(e);
        }
    }
}

To reproduce:

public Form1()
{
    InitializeComponent();

    this.radVirtualGrid1.ColumnCount = 5;
    this.radVirtualGrid1.RowCount = 20;
    this.radVirtualGrid1.CellValueNeeded += radVirtualGrid1_CellValueNeeded;
}

private void radVirtualGrid1_CellValueNeeded(object sender, Telerik.WinControls.UI.VirtualGridCellValueNeededEventArgs e)
{
    e.Value = "Data" + e.RowIndex + "." + e.ColumnIndex;
}

Workaround: use a custom ScrollableVirtualCellsContainer:

public Form1()
{
    InitializeComponent();
    this.radVirtualGrid1.CreateRowElement += radVirtualGrid1_CreateRowElement;
    this.radVirtualGrid1.ColumnCount = 5;
    this.radVirtualGrid1.RowCount = 20;
    this.radVirtualGrid1.CellValueNeeded += radVirtualGrid1_CellValueNeeded;
}

private void radVirtualGrid1_CreateRowElement(object sender, VirtualGridCreateRowEventArgs e)
{
    e.RowElement = new CustomVirtualGridRowElement();
}

private void radVirtualGrid1_CellValueNeeded(object sender, Telerik.WinControls.UI.VirtualGridCellValueNeededEventArgs e)
{
    e.Value = "Data" + e.RowIndex + "." + e.ColumnIndex;
}

public class CustomVirtualGridRowElement : VirtualGridRowElement
{
    public CustomVirtualGridRowElement()
        : base()
    {
        FieldInfo fi = typeof(VirtualGridRowElement).GetField("scrollableColumns", BindingFlags.NonPublic | BindingFlags.Instance);
        ScrollableVirtualCellsContainer scrollableColumns = fi.GetValue(this) as ScrollableVirtualCellsContainer;
        CustomScrollableVirtualCellsContainer newScrollableColumns = new CustomScrollableVirtualCellsContainer();
        FieldInfo fi2 = typeof(ScrollableVirtualCellsContainer).GetField("rowElement", BindingFlags.NonPublic | BindingFlags.Instance);
        fi2.SetValue(newScrollableColumns, this);
        this.Children.Remove(scrollableColumns);
        this.Children.Insert(1, newScrollableColumns);
    }
}

public class CustomScrollableVirtualCellsContainer : ScrollableVirtualCellsContainer
{
    protected override SizeF MeasureElementCore(RadElement element, SizeF availableSize)
    {
        int scrollOffset = 0;

        if (this.RowElement.TableElement.ColumnScroller.ScrollMode == ItemScrollerScrollModes.Discrete)
        {
            for (int i = 0; i < this.RowElement.TableElement.ColumnScroller.Scrollbar.Value; i++)
            {
                scrollOffset += this.RowElement.TableElement.ColumnScroller.GetScrollHeight(
                    this.RowElement.TableElement.ViewInfo.ColumnsViewState.GetItemSize(i));
            }
        }
        else
        {
            scrollOffset = this.RowElement.TableElement.HScrollBar.Value;
        }

        VirtualGridCellElement cellElement = element as VirtualGridCellElement;

        int itemOffset = this.RowElement.ViewInfo.ColumnsViewState.GetItemOffset(cellElement.Data);
        int itemWidth = this.RowElement.ViewInfo.ColumnsViewState.GetItemSize(cellElement.Data);

        element.Measure(availableSize);

        if (itemOffset + itemWidth > scrollOffset && itemOffset < scrollOffset)
        {
            return new SizeF((itemOffset + itemWidth) - scrollOffset, element.DesiredSize.Height);
        }

        return element.DesiredSize;
    }
}

Please refer to the attached sample project.

To reproduce:
- Subscribe to the SelectionChanged event.
- Move the mouse while holding the left button.
- The event is fire multiple times.

Workaround:
VirtualGridCellInfo prevCell = null;
private void OnGridSelectionChanged(object sender, EventArgs e)
{
    if (prevCell != gvItems.CurrentCell)
    {
        _selectionChangedCount++;
    
        tbLog.AppendText("Selection changed " + _selectionChangedCount + "\r\n");
    }
    prevCell = gvItems.CurrentCell;
}

Please refer to the attached gif file demonstrating how to reproduce the issue with the Demo application.

Workaround:

Public Class Form1
    Sub New()
        InitializeComponent()
        Me.RadVirtualGrid1.AllowAddNewRow = False
        Me.RadVirtualGrid1.AllowFiltering = False
        Me.RadVirtualGrid1.ColumnCount = 5
        Me.RadVirtualGrid1.MasterViewInfo.RegisterCustomColumn(3)     
        Me.RadVirtualGrid1.RowCount = 1
    End Sub
    Private Sub RadVirtualGrid1_CreateCellElement(sender As Object, e As VirtualGridCreateCellEventArgs) Handles RadVirtualGrid1.CreateCellElement
        Dim columnIndex As Integer = e.ColumnIndex
        If e.ViewInfo.ColumnsViewState.TopPinnedItems.Count > 0 AndAlso e.ColumnIndex - 1 < e.ViewInfo.ColumnsViewState.TopPinnedItems.Count Then
            columnIndex = e.ViewInfo.ColumnsViewState.TopPinnedItems(e.ColumnIndex - 1)
        End If

        If columnIndex = 3 AndAlso e.RowIndex >= 0 Then
            e.CellElement = New RedCellElement()
        End If
    End Sub

    Private Sub RadVirtualGrid1_CellValueNeeded(sender As Object, e As VirtualGridCellValueNeededEventArgs) Handles RadVirtualGrid1.CellValueNeeded
        e.Value = String.Format("{0}x{1}", e.RowIndex, e.ColumnIndex)
    End Sub
End Class

Class RedCellElement
    Inherits VirtualGridCellElement

    Sub New()
        MyBase.New()
    End Sub

    Private checkBox As RadCheckBoxElement
    Protected Overrides Sub CreateChildElements()
        MyBase.CreateChildElements()
        Me.checkBox = New RadCheckBoxElement()
        Me.Children.Add(Me.checkBox)
    End Sub

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

    Public Overrides Function IsCompatible(data As Integer, context As Object) As Boolean
        Dim headerRow As VirtualGridHeaderRowElement = TryCast(context, VirtualGridHeaderRowElement)
        If data >= 3 AndAlso headerRow Is Nothing Then
            Return True
        Else
            Return False
        End If
    End Function

    Protected Overrides Sub UpdateInfo(args As VirtualGridCellValueNeededEventArgs)
        MyBase.UpdateInfo(args)
        Me.NumberOfColors = 1
        Me.DrawFill = True
        Me.BackColor = Color.Red
    End Sub

End Class

To reproduce:
public RadForm1()
{
    InitializeComponent();
    radVirtualGrid1.CellValueNeeded += RadVirtualGrid1_CellValueNeeded;
    radVirtualGrid1.ColumnCount = 100;
    radVirtualGrid1.RowCount = 50;
    radVirtualGrid1.AutoSizeColumnsMode = Telerik.WinControls.UI.VirtualGridAutoSizeColumnsMode.Fill;
    radVirtualGrid1.SelectionMode = Telerik.WinControls.UI.VirtualGridSelectionMode.CellSelect;
}

private void RadVirtualGrid1_CellValueNeeded(object sender, Telerik.WinControls.UI.VirtualGridCellValueNeededEventArgs e)
{
    e.Value = string.Format("Col {0} Row{1}", e.ColumnIndex, e.RowIndex);
}

To reproduce:
public RadForm1()
{
    InitializeComponent();
    radVirtualGrid1.CellValueNeeded += RadVirtualGrid1_CellValueNeeded;
    radVirtualGrid1.ColumnCount = 20;
    radVirtualGrid1.RowCount = 1000;
    radVirtualGrid1.AutoSizeColumnsMode = Telerik.WinControls.UI.VirtualGridAutoSizeColumnsMode.Fill;
}

private void RadVirtualGrid1_CellValueNeeded(object sender, Telerik.WinControls.UI.VirtualGridCellValueNeededEventArgs e)
{
    e.Value = string.Format("Col {0} Row{1}", e.ColumnIndex, e.RowIndex);
}

- Press and hold the down key, you will notice that the selection freezes after some time and is updated after the key is released.  

A newly created grid has its CurrentCell property initialized to Nothing. Pressing [PageDown] or any other key that causes scrolling will crash.

Steps to Reproduce:
1. Launch application
2. Press [PageDown]

 Sub New() 
        InitializeComponent()

        Me.RadVirtualGrid1.ColumnCount = 50
        Me.RadVirtualGrid1.RowCount = 100
        AddHandler Me.RadVirtualGrid1.CellValueNeeded, AddressOf RadVirtualGrid1_CellValueNeeded
    End Sub

    Private Sub RadVirtualGrid1_CellValueNeeded(sender As Object, e As Telerik.WinControls.UI.VirtualGridCellValueNeededEventArgs)
        e.Value = e.ColumnIndex
    End Sub

Workaround: 

Me.RadVirtualGrid1.CurrentCell = New Telerik.WinControls.UI.VirtualGridCellInfo(0, 0, Me.RadVirtualGrid1.MasterViewInfo)

Description
When the RowCount/ColumnCount amount is decreased, and there are resized rows/column, the scrollable area get invalid.

Steps to reproduce:
  RadVirtualGrid radVirtualGrid1 = new RadVirtualGrid();

        public Form1()
        {
            InitializeComponent();

            radVirtualGrid1.Parent = this;
            radVirtualGrid1.Dock = DockStyle.Fill;
            this.Controls.SetChildIndex(radVirtualGrid1, 0);
            button1.BringToFront();

            radVirtualGrid1.RowCount = 1000;
            radVirtualGrid1.ColumnCount = 50;

            radVirtualGrid1.CellValueNeeded += radVirtualGrid1_CellValueNeeded;


            for (int i = 0; i < 50; i++)
            {
                radVirtualGrid1.MasterViewInfo.SetRowHeight(i, 30);

                radVirtualGrid1.MasterViewInfo.SetColumnWidth(i, 60);
            }
        }

        void radVirtualGrid1_CellValueNeeded(object sender, VirtualGridCellValueNeededEventArgs e)
        {
            e.Value = e.RowIndex + " " + e.ColumnIndex;
        }


        private void button1_Click(object sender, EventArgs e)
        {
            radVirtualGrid1.RowCount = 20;
            radVirtualGrid1.ColumnCount = 20;
        }

To reproduce:
Public Class Form1
    Private Sub Form1_Load(sender As Object, e As EventArgs) Handles MyBase.Load
        Me.RadVirtualGrid1.SelectionMode = Telerik.WinControls.UI.VirtualGridSelectionMode.CellSelect
        Me.RadVirtualGrid1.MultiSelect = True
 
        Me.RadVirtualGrid1.RowCount = 10
        Me.RadVirtualGrid1.ColumnCount = 5
 
        Me.RadVirtualGrid1.SelectAll()
 
        Debug.Assert(Me.RadVirtualGrid1.Selection.MaxColumnIndex = Me.RadVirtualGrid1.ColumnCount - 1)
        Debug.Assert(Me.RadVirtualGrid1.Selection.MaxRowIndex = Me.RadVirtualGrid1.RowCount - 1)
    End Sub
End Class


Workaround:
Private Sub RadVirtualGrid1_SelectionChanging(ByVal sender As Object, ByVal e As VirtualGridSelectionChangingEventArgs)
	If e.SelectionAction = VirtualGridSelectionAction.SelectAll Then
		e.Cancel = True
		radVirtualGrid1.VirtualGridElement.Selection.BeginSelection(0,0, radVirtualGrid1.MasterViewInfo, False)
		radVirtualGrid1.VirtualGridElement.Selection.ExtendCurrentRegion(radVirtualGrid1.RowCount-1,radVirtualGrid1.ColumnCount -1)
	End If
End Sub

To reproduce:

1. Select multiple cells in RadVirtualGrid
2. Right-click one of the selected cells. The expected behavior is to open the context menu in order to allow copy operation and keep the selected cells. However, the obtained result is that the selected cells are cleared.

Workaround:
Me.RadVirtualGrid1.VirtualGridElement.InputBehavior = New CustoVirtualGridInputBehavior(Me.RadVirtualGrid1.VirtualGridElement)
AddHandler Me.RadVirtualGrid1.SelectionChanging, AddressOf SelectionChanging

Private Sub SelectionChanging(sender As Object, e As Telerik.WinControls.UI.VirtualGridSelectionChangingEventArgs)
    If Me.RadVirtualGrid1.VirtualGridElement.Tag = "CancelSelectionChange" Then
        e.Cancel = True
        Me.RadVirtualGrid1.VirtualGridElement.Tag = Nothing
    End If

End Sub

Public Class CustoVirtualGridInputBehavior
Inherits VirtualGridInputBehavior
    Public Sub New(gridElement As RadVirtualGridElement)
        MyBase.New(gridElement)

    End Sub

    Public Overrides Function HandleMouseDown(args As MouseEventArgs) As Boolean
        If args.Button = MouseButtons.Right Then
            Dim cell As VirtualGridCellElement = TryCast(Me.GridElement.ElementTree.GetElementAtPoint(args.Location), VirtualGridCellElement)
            If cell IsNot Nothing AndAlso cell.IsSelected Then
                Me.GridElement.Tag = "CancelSelectionChange"
            End If

        End If 
        Return MyBase.HandleMouseDown(args)
    End Function 
End Class

When a row/column is resized, and we scroll to the last row/column, they are cut off, due to an error in the item spacing calculation

To reproduce:
            radVirtualGrid1.RowCount = 1000;
            radVirtualGrid1.ColumnCount = 50;

            radVirtualGrid1.MasterViewInfo.SetRowHeight(50, 30);

            for (int i = 0; i < 50; i++)
            {
                radVirtualGrid1.MasterViewInfo.SetColumnWidth(i, 60);
            }

        void radVirtualGrid1_CellValueNeeded(object sender, VirtualGridCellValueNeededEventArgs e)
        {
            e.Value = e.RowIndex + " " + e.ColumnIndex;
        }

Workaround: set the RadVirtualGrid.TableElement.ColumnWidth property to 60 instead of using the SetColumnWidth method and set the radVirtualGrid1.MasterViewInfo.RowSpacing to 0 to overcome the row issue.