Silverlight DataGrid with Combobox rendering problem

Question

As you know that we have already implemented Silverlight application in our application, we need your help to resolve one issue that currently we are facing, actually this is a production bug. We are using Silverligh3 latest version. In our Silverlight
page we have one data grid and inside that we have one dropdown list as a column and others are textboxes. The dropdown lists are attached with the data context using two way binding. Now in order to simulate the issue, add multiple rows (using front-end ‘Add
Row’ custom button) on that data grid at runtime so that scrollbar is coming on the data grid, select data from combo boxes from first two rows, and after scrolling up and  down for 2-3times causing combo boxes data disappeared from selected ones and it is
appearing on rows where no values are selected earlier. But if you reload the page then everything is coming as before. And we have checked that the data context object is purely ok and this is a layout refreshing problem only. I do not know whether this is
a bug of Silverlight or not, but as this is a production issue, I need your help urgently.

Answer 1

Thanks for your reply. I have simulated the problem in a test page which I have uploaded in this forum. As per your suggestion I have added the code but I doubt whether I have
done it properly. Please check the code as after incorporating the StackPanel also it behaves the same way. You will find the source code for Page1.xaml and the Page1.xaml.vb below in this page. Please review and let me know why virtualization is still active. 
Page1.xaml
<navigation:Page x:Class="SLClientApp.Page1"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
xmlns:data="clr-namespace:System.Windows.Controls;assembly=System.Windows.Controls.Data"
mc:Ignorable="d"
xmlns:navigation="clr-namespace:System.Windows.Controls;assembly=System.Windows.Controls.Navigation"
d:DesignWidth="640" d:DesignHeight="480"
Title="Page1 Page">
<Grid x:Name="LayoutRoot">
<data:DataGrid Margin="18,0,18,18" VirtualizingStackPanel.VirtualizationMode="Standard" x:Name="dtgTestGrid" ItemsSource="{Binding Mode=TwoWay, Path=TestDataCol}" AutoGenerateColumns="False" >
<data:DataGrid.Columns>
<data:DataGridTemplateColumn Header="Test Combo">
<data:DataGridTemplateColumn.CellTemplate>
<DataTemplate>
<ComboBox VirtualizingStackPanel.VirtualizationMode="Standard" Loaded="SetDataForCombo" x:Name="cmbTestCombo" Height="24" Width="120" >
<ComboBox.ItemsPanel>
<ItemsPanelTemplate>
<StackPanel/>
</ItemsPanelTemplate>
</ComboBox.ItemsPanel>
</ComboBox>
</DataTemplate>
</data:DataGridTemplateColumn.CellTemplate>
</data:DataGridTemplateColumn>
</data:DataGrid.Columns>
</data:DataGrid>
</Grid>
</navigation:Page>
Page1.xaml.vb
Imports System.Collections.ObjectModel
Partial Public Class Page1
Inherits Page
Public objTestClassCol As New TestClassCol
Public Sub New()
InitializeComponent()
End Sub
'Executes when the user navigates to this page.
Protected Overrides Sub OnNavigatedTo(ByVal e As System.Windows.Navigation.NavigationEventArgs)
End Sub
Private Sub SetDataForCombo(ByVal sender As System.Object, ByVal e As System.Windows.RoutedEventArgs)
For iCount = 1 To 5
CType(sender, System.Windows.Controls.ComboBox).Items.Add("Value" + CStr(iCount))
Next iCount
End Sub
Private Sub Page1_Loaded(ByVal sender As Object, ByVal e As System.Windows.RoutedEventArgs) Handles Me.Loaded
LayoutRoot.DataContext = objTestClassCol
For iCount = 1 To 30
objTestClassCol.TestDataCol.Add(New TestClass)
Next
End Sub
End Class
Public Class TestClass
Private strLVTestData As String
Public Property TestData() As String
Get
Return strLVTestData
End Get
Set(ByVal value As String)
strLVTestData = value
End Set
End Property
End Class
Public Class TestClassCol
Private objLVTestData As New ObservableCollection(Of TestClass)
Public Property TestDataCol() As ObservableCollection(Of TestClass)
Get
Return objLVTestData
End Get
Set(ByVal value As ObservableCollection(Of TestClass))
objLVTestData = value
End Set
End Property
End Class