Steps to reproduce:
1. Add RadGridView and populate with data 
2. Show the context menu many times. Sometimes the popup is not shown correctly.
If you click again, the popup is visible correctly.

Workaround: 
Set the AnimationEnabled property to false or the AnimationType property to None. Here is the code snippet how can be achieve it: 

//Set the AnimationEnabled to false
void radGridView1_ContextMenuOpening(object sender, Telerik.WinControls.UI.ContextMenuOpeningEventArgs e)
{
    RadDropDownMenu contextMenu = e.ContextMenu as RadDropDownMenu;
    contextMenu.AnimationEnabled = false;
}
 
//Set the AnimationType to None
void radGridView1_ContextMenuOpening(object sender, Telerik.WinControls.UI.ContextMenuOpeningEventArgs e)
{
    RadDropDownMenu contextMenu = e.ContextMenu as RadDropDownMenu;
    contextMenu.AnimationType = PopupAnimationTypes.None;
}

How to reproduce: check the attached video

Workaround: disable pinning of the last row when it is in a group
public partial class Form1 : Form
{
    DataTable dt;

    public Form1()
    {
        InitializeComponent();

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

        this.radGridView1.ContextMenuOpening += RadGridView1_ContextMenuOpening;
    }

    private void RadGridView1_ContextMenuOpening(object sender, Telerik.WinControls.UI.ContextMenuOpeningEventArgs e)
    {
        GridRowHeaderCellElement header = e.ContextMenuProvider as GridRowHeaderCellElement;
        if (header != null && header.RowInfo.Group != null)
        {
            var notPinned = header.RowInfo.Group.GroupRow.ChildRows.Where(r => r.IsPinned == false).ToList();
            if (notPinned.Count <= 1)
            {
                e.ContextMenu.Items.RemoveAt(0);
                e.ContextMenu.Items.RemoveAt(0);
            }
        }
    }

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

        return dt;
    }
}

How to reproduce: 
public partial class Form1 : Form
{
    DataTable dt;

    public Form1()
    {
        InitializeComponent();

        this.radGridView1.DataSource = this.GetData();
        this.radGridView1.AutoSizeColumnsMode = Telerik.WinControls.UI.GridViewAutoSizeColumnsMode.Fill;
    }

    private void radButton1_Click(object sender, EventArgs e)
    {
        dt.Clear();
        for (int i = 0; i < 100; i++)
        {
            dt.Rows.Add(i, "New Name " + i, DateTime.Now.AddDays(i), i % 2 == 0);
        }
    }

    private DataTable GetData()
    {
        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 < 500; i++)
        {
            dt.Rows.Add(i, "Name " + i, DateTime.Now.AddDays(i), i % 2 == 0);
        }

        return dt;
    }
}

Workaround:
public partial class Form1 : Form
{
    DataTable dt;

    public Form1()
    {
        InitializeComponent();

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

        this.radGridView1.RowsChanged += RadGridView1_RowsChanged;
    }

    private void RadGridView1_RowsChanged(object sender, Telerik.WinControls.UI.GridViewCollectionChangedEventArgs e)
    {
        if (e.Action == Telerik.WinControls.Data.NotifyCollectionChangedAction.Reset)
        {
            this.radGridView1.MasterView.PinnedRows.GetType().GetMethod("Clear", System.Reflection.BindingFlags.Instance | System.Reflection.BindingFlags.NonPublic)
                .Invoke(this.radGridView1.MasterView.PinnedRows, new object[] { });
        }
    }

    private void radButton1_Click(object sender, EventArgs e)
    {
        dt.Clear();
        for (int i = 0; i < 100; i++)
        {
            dt.Rows.Add(i, "New Name " + i, DateTime.Now.AddDays(i), i % 2 == 0);
        }
    }

    private DataTable GetData()
    {
        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 < 500; i++)
        {
            dt.Rows.Add(i, "Name " + i, DateTime.Now.AddDays(i), i % 2 == 0);
        }

        return dt;
    }
}

Workaround:

AddHandler Me.RadGridView1.CreateCompositeFilterDialog, AddressOf CreateCompositeFilterDialog

Private Sub CreateCompositeFilterDialog(sender As Object, e As GridViewCreateCompositeFilterDialogEventArgs)
    Dim dialog As New CompositeDataFilterForm()
    AddHandler dialog.DataFilter.EditorRequired, AddressOf EditorRequired
    e.Dialog = dialog
End Sub

Private Sub EditorRequired(sender As Object, e As TreeNodeEditorRequiredEventArgs)
    Dim filterNode As DataFilterCriteriaNode = TryCast(e.Node, DataFilterCriteriaNode)
    If filterNode IsNot Nothing AndAlso filterNode.PropertyName = "BASE_NULL_DATE" AndAlso TypeOf sender Is DataFilterValueEditorElement Then
        e.Editor = New TreeViewDateTimeEditor()
    End If
End Sub

To reproduce: please refer to the attached sample project and follow the described steps in the .doc file located in the zip. 

Workaround:

        protected override void OnActivated(EventArgs e)
        {
            base.OnActivated(e);
            this.ActiveControl = this.gvDetails;
        }

       private void gvDetails_LostFocus(object sender, EventArgs e)
        { 
            ((ComponentBehavior)gvDetails.Behavior).GetType().GetProperties(BindingFlags.NonPublic | BindingFlags.Instance);
            PropertyInfo barProperty = ((ComponentBehavior)gvDetails.Behavior).GetType().GetProperty("ScreenPresenter", 
                BindingFlags.NonPublic | BindingFlags.Instance);
            Form screenTip = barProperty.GetValue(((ComponentBehavior)gvDetails.Behavior), null) as Form;
                    
            screenTip.Hide();
        }

To reproduce: please open the attached sample project and follow the steps illustrated in the attached gif file.

Workaround: 
1. You still can scroll while dragging a row by using the mouse wheel.

2. Use the grid in unbound mode and set the AllowRowReorder property to true instead of using a custom RadDragDropService.

3. Use a custom drag and drop service:

public class CustomDragDropService : RadGridViewDragDropService
{
    public CustomDragDropService(RadGridViewElement gridViewElement) : base(gridViewElement)
    {
    }

    public override string Name
    {
        get
        {
            return typeof(RadGridViewDragDropService).Name;
        }
    }

    protected override void HandleMouseMove(System.Drawing.Point mousePosition)
    {
        base.HandleMouseMove(mousePosition);
        System.Drawing.Point location = this.GridViewElement.ElementTree.Control.PointToClient(Control.MousePosition);
        GridTableElement tableElement = this.GetTableElementAtPoint(location);

        ISupportDrag supportDrag = this.Context as ISupportDrag;
        object dataContext = supportDrag.GetDataContext();

        if (this.AllowAutoScrollRowsWhileDragging && dataContext == null)
        {
            ScrollRows(tableElement, location);
        }
    }

    private void ScrollRows(GridTableElement tableElement, System.Drawing.Point location)
    {
        ScrollableRowsContainerElement scrollableRows = tableElement.ViewElement.ScrollableRows;
        RadScrollBarElement vScrollbar = GetVeritcalScrollbar(tableElement);
        System.Drawing.Rectangle containerBounds = scrollableRows.ControlBoundingRectangle;

        if (containerBounds.Contains(location) ||
            location.X < containerBounds.X ||
            location.X > containerBounds.Right)
        {
            return;
        }

        int delta = 0;

        if (location.Y > containerBounds.Bottom)
        {
            delta = location.Y - containerBounds.Bottom;
        }
        else if (location.Y < containerBounds.Y)
        {
            delta = location.Y - containerBounds.Y;
        }

        if (delta != 0 && vScrollbar.Visibility == ElementVisibility.Visible)
        {
            vScrollbar.Value = ClampValue(vScrollbar.Value + delta,
                vScrollbar.Minimum,
                vScrollbar.Maximum - vScrollbar.LargeChange + 1);
        }
    }

    private int ClampValue(int value, int minimum, int maximum)
    {
        if (value < minimum)
        {
            return minimum;
        }
        if (maximum > 0 && value > maximum)
        {
            return maximum;
        }
        return value;
    }

    private RadScrollBarElement GetVeritcalScrollbar(GridTableElement tableElement)
    {
        if (GridViewElement.UseScrollbarsInHierarchy)
        {
            return tableElement.VScrollBar;
        }
        return GridViewElement.TableElement.VScrollBar;
    }
}

public RadForm1()
{
    InitializeComponent();
    CustomDragDropService customService = new CustomDragDropService(radGridView1.GridViewElement);
    radGridView1.GridViewElement.RegisterService(customService);
}

To reproduce: please refer to the attached sample project. Note that the CellValidating event is fired twice even without showing a RadMessageBox. With an MS Button the problem is not reproducible.

Workaround: use the RadGridView.ValueChanging event to perform validation while the user is typing in the active editor:
 private void radGridView1_ValueChanging(object sender, Telerik.WinControls.UI.ValueChangingEventArgs e)
 {
   if (((string)e.NewValue) != "x")
     {
         RadMessageBox.Show("Only 'x' allowed!");
         e.Cancel = true;
     }
 }

To reproduce: please refer to the attached sample project and gif file.

Workaround: use the CellFormatting event and apply the light orange BackColor for the selected cells belonging to the current pinned column:

private void radGridView1_CellFormatting(object sender, Telerik.WinControls.UI.CellFormattingEventArgs e)
{
    if (e.Row.IsSelected && e.Column.IsCurrent)
    {
        e.CellElement.BackColor = Color.FromArgb(255, 231, 174);
    }
    else
    { 
        e.CellElement.ResetValue(LightVisualElement.BackColorProperty, ValueResetFlags.Local);
    }
}

Use attached project to reproduce!

Another case is when the font size is changed from the settings dialog - in this case, the row height is not adjusted.

Workaround:
Use the following custom print style:

class MyTableViewDefinitionPrintRenderer : TableViewDefinitionPrintRenderer
{
    public MyTableViewDefinitionPrintRenderer(RadGridView grid) : base(grid)
    {

    }
    protected override int GetDataRowHeight(GridViewRowInfo row, TableViewRowLayoutBase rowLayout)
    {
        int result = base.GetDataRowHeight(row, rowLayout);

        int newHeight = 0;

        if (!(row is GridViewGroupRowInfo))
        {
            foreach (GridViewColumn col in row.ViewTemplate.Columns)
            {
                if (col is GridViewRowHeaderColumn || col is GridViewIndentColumn || !col.IsVisible)
                {
                    continue;
                }

                string value = row.Cells[col.Name].Value.ToString();

                TableViewCellArrangeInfo info = ((TableViewRowLayout)rowLayout).LayoutImpl.GetArrangeInfo(col);
                float cellWidth = (float)info.CachedWidth;
                int currentHeight = TextRenderer.MeasureText(value, this.GridView.PrintStyle.CellFont, new Size((int)cellWidth, 0), TextFormatFlags.WordBreak).Height + this.GridView.Font.Height *4;

                newHeight = Math.Max(newHeight, currentHeight);

            }
        }


        return Math.Max(newHeight, result); 
    }
}
class MyPrintStyle :GridPrintStyle
{
    protected override BaseGridPrintRenderer InitializePrintRenderer(RadGridView grid)
    {
        return new MyTableViewDefinitionPrintRenderer(grid);
    }
}

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: run the attached sample project and you will notice that the top border is missing as it is illustrated in the screenshot. 

Workaround:  set the RadGridView.AutoSizeRows property to false.

To reproduce:
public RadForm1()
{
    InitializeComponent();
    radGridView1.DataSource = GetTable();
    GridViewSummaryItem summaryItem = new GridViewSummaryItem();
    summaryItem.Name = "Name";
    summaryItem.Aggregate = GridAggregateFunction.Count;
    GridViewSummaryRowItem summaryRowItem = new GridViewSummaryRowItem();
    summaryRowItem.Add(summaryItem);
    this.radGridView1.SummaryRowsBottom.Add(summaryRowItem);

    radGridView1.ViewCellFormatting += RadGridView1_ViewCellFormatting;
}

private void RadGridView1_ViewCellFormatting(object sender, CellFormattingEventArgs e)
{
    if (e.CellElement is GridSummaryCellElement)
    {
        e.CellElement.DrawFill = true;
        e.CellElement.GradientStyle = Telerik.WinControls.GradientStyles.Solid;
        e.CellElement.BackColor = Color.Red;

    }
    else
    {
        e.CellElement.ResetValue(LightVisualElement.BackColorProperty, Telerik.WinControls.ValueResetFlags.Local);
        e.CellElement.ResetValue(LightVisualElement.GradientStyleProperty, Telerik.WinControls.ValueResetFlags.Local);
        e.CellElement.ResetValue(LightVisualElement.DrawFillProperty, Telerik.WinControls.ValueResetFlags.Local);
    }
}

protected override void OnLoad(EventArgs e)
{
    base.OnLoad(e);
    radGridView1.MasterView.SummaryRows[0].PinPosition = PinnedRowPosition.Bottom;
}

static DataTable GetTable()
{

    DataTable table = new DataTable();
    table.Columns.Add("Dosage", typeof(int));
    table.Columns.Add("Drug", typeof(string));
    table.Columns.Add("Name", typeof(string));
    table.Columns.Add("Date", typeof(DateTime));

    for (int i = 0; i < 5; i++)
    {


        table.Rows.Add(25, "Indocin", "David", DateTime.Now);
        table.Rows.Add(50, "Enebrel", "Sam", DateTime.Now);
        table.Rows.Add(10, "Hydralazine", "Christoff", DateTime.Now);
        table.Rows.Add(21, "Combivent", "Janet", DateTime.Now);
        table.Rows.Add(100, "Dilantin", "Melanie", DateTime.Now);
    }
    return table;
}
private void radButton1_Click(object sender, EventArgs e)
{

    radGridView1.SaveLayout("test.xml");
}

private void radButton2_Click(object sender, EventArgs e)
{
    radGridView1.SummaryRowsBottom.Clear();
    radGridView1.SummaryRowsTop.Clear();
    radGridView1.LoadLayout("test.xml");
}

To reproduce:
this.radGridView1.MultiSelect = true;
this.radGridView1.SelectionMode = Telerik.WinControls.UI.GridViewSelectionMode.CellSelect;

radGridView1.ShowRowHeaderColumn = false;

Then select the cells in the firs column by dragging the mouse.

Workaround:
// radGridView1.ShowRowHeaderColumn = false;
radGridView1.TableElement.RowHeaderColumnWidth = 0;

Workaround: handle the GridViewPdfExport.CellFormatting event and apply the column`s format string

Public Class Form1
    Sub New()

        InitializeComponent()

        Me.RadGridView1.DataSource = Me.GetData()
        Me.RadGridView1.AutoSizeColumnsMode = Telerik.WinControls.UI.GridViewAutoSizeColumnsMode.Fill

        Dim decimalColumn = DirectCast(Me.RadGridView1.Columns("Money"), GridViewDecimalColumn)
        decimalColumn.DecimalPlaces = 0
        decimalColumn.FormatString = "{0:C0}"

        Dim dateTimeColumn = DirectCast(Me.RadGridView1.Columns("Date"), GridViewDateTimeColumn)
        dateTimeColumn.FormatString = "{0:D}"

    End Sub

    Private Function GetData() As Object
        Dim dataTable As New DataTable()
        dataTable.Columns.Add("Id", GetType(Integer))
        dataTable.Columns.Add("Name", GetType(String))
        dataTable.Columns.Add("Money", GetType(Decimal))
        dataTable.Columns.Add("Date", GetType(DateTime))

        For i As Integer = 0 To 999
            dataTable.Rows.Add(i, "Name " & i, i * 10, DateTime.Now.AddDays(i))
        Next

        Return dataTable
    End Function

    Private Sub RadButton1_Click(sender As Object, e As EventArgs) Handles RadButton1.Click
        Dim pdfExporter = New GridViewPdfExport(Me.RadGridView1)
        AddHandler pdfExporter.CellFormatting, AddressOf pdfExporter_CellFormatting
        Dim renderer = New PdfExportRenderer()
        Dim fileName As String = "..\..\exported-grid.pdf"

        pdfExporter.RunExport(fileName, renderer)
    End Sub

    Private Sub pdfExporter_CellFormatting(sender As Object, e As PdfExportCellFormattingEventArgs)
        If e.RowIndex > -1 Then
            e.CellElement.Text = TryCast(RadDataConverter.Instance.Format(e.Row.Cells(e.ColumnIndex).Value, GetType(String), e.Column), String)
        End If
    End Sub

End Class

How to reproduce:
public partial class Form1 : Form
{
    private RadGridView grid = new RadGridView();

    public Form1()
    {
        InitializeComponent();

        Controls.Add(grid);
        grid.Dock = DockStyle.Fill;
        grid.DataSource = this.GetData();
    }

    private object GetData()
    {
        DataTable dataTable = new DataTable();
        dataTable.Columns.Add("Id", typeof(int));
        dataTable.Columns.Add("Name", typeof(string));
        dataTable.Columns.Add("Checked", typeof(bool));

        for (int i = 0; i < 200; i++)
        {
            dataTable.Rows.Add(i, "Name " + i, i % 2 == 0);
        }

        return dataTable;
    }

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

        grid.AutoSizeColumnsMode = GridViewAutoSizeColumnsMode.Fill;

        GridViewCheckBoxColumn checkBoxColumn = (GridViewCheckBoxColumn)grid.Columns["Checked"];
        checkBoxColumn.EnableHeaderCheckBox = true;
        checkBoxColumn.SortOrder = RadSortOrder.Ascending;
    }
}

Workaround: 
public partial class Form1 : Form
{
    private RadGridView grid = new RadGridView();

    public Form1()
    {
        InitializeComponent();

        Controls.Add(grid);
        grid.Dock = DockStyle.Fill;
        grid.DataSource = this.GetData();

        grid.MouseDown += grid_MouseDown;
        grid.MouseUp += grid_MouseUp;
    }

    private object GetData()
    {
        DataTable dataTable = new DataTable();
        dataTable.Columns.Add("Id", typeof(int));
        dataTable.Columns.Add("Name", typeof(string));
        dataTable.Columns.Add("Checked", typeof(bool));

        for (int i = 0; i < 200; i++)
        {
            dataTable.Rows.Add(i, "Name " + i, i % 2 == 0);
        }

        return dataTable;
    }

    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)
        {
            sw = new Stopwatch();
            sw.Start();
            grid.BeginUpdate();
        }
    }

    Stopwatch sw;
    private void grid_MouseUp(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.EndUpdate();
            sw.Stop();
            Console.WriteLine(sw.ElapsedMilliseconds);
        }
    }

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

        grid.AutoSizeColumnsMode = GridViewAutoSizeColumnsMode.Fill;

        GridViewCheckBoxColumn checkBoxColumn = (GridViewCheckBoxColumn)grid.Columns["Checked"];
        checkBoxColumn.EnableHeaderCheckBox = true;
        checkBoxColumn.SortOrder = RadSortOrder.Ascending;
    }
}

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();
}

To reproduce
- Start the attached project press "Add in code"  then "Save"
- The row is still not added to the underlying data table. 

Workaround:
TryCast(Me.radGridView1.Rows.Last().DataBoundItem, IEditableObject).EndEdit()