WPF combobox items list/context menu rendered behind parent - wpf

Question

If the ItemSource property of a combo box has been set, why could clicking on the drop down botton of a combobox not display the list of available items? This may be related but within the same control, any context menu is displayed behind the user control:
The XAML for this control is as follows:
<Border Name="Border" Padding="5">
<ScrollViewer VerticalScrollBarVisibility="Auto">
<ScrollViewer.Resources>
<Style TargetType="{x:Type CheckBox}">
<Setter Property="Padding" Value="8,0,0,0"/>
<Setter Property="VerticalAlignment" Value="Center"/>
</Style>
</ScrollViewer.Resources>
<Grid>
<Grid.RowDefinitions>
<RowDefinition Height="Auto"/>
<RowDefinition Height="Auto"/>
<RowDefinition Height="*"/>
</Grid.RowDefinitions>
<Grid Margin="5,4,0,4" >
<Grid.ColumnDefinitions>
<ColumnDefinition Width="20" />
<ColumnDefinition SharedSizeGroup="labelColumn1" />
<ColumnDefinition SharedSizeGroup="labelColumn2" />
<ColumnDefinition SharedSizeGroup="dataEntryColumn" />
<ColumnDefinition Width="30"/>
<ColumnDefinition SharedSizeGroup="labelColumn2"/>
<ColumnDefinition SharedSizeGroup="dataEntryColumn" />
<ColumnDefinition MaxWidth="0" />
</Grid.ColumnDefinitions>
<Grid.RowDefinitions>
<RowDefinition MinHeight="23" />
<RowDefinition MinHeight="23" />
<RowDefinition Height="Auto" />
<RowDefinition Height="Auto"/>
</Grid.RowDefinitions>
<TextBlock Text="Geometry Type" VerticalAlignment="Center" Grid.Column="1" Grid.Row="0"/>
<ComboBox Grid.Column="3" Margin="6,1,0,1" Grid.Row="0" Width="150"
Name="cmboGeometryTypes"
SelectedItem="{Binding GeometryType, Mode=TwoWay}"
HorizontalAlignment="Left"
DisplayMemberPath="Name"
Grid.ColumnSpan="1"
/>
<TextBlock Text="Symbol Type"
Grid.Column="1" Grid.Row="1" VerticalAlignment="Center"/>
<ComboBox
Name="cmboSymbolEditors"
SelectedItem="{Binding SymbolEditorViewModel, Mode=TwoWay}"
HorizontalAlignment="Left"
DisplayMemberPath="Alias"
Width="150"
Grid.Column="3"
Margin="6,1,0,1"
Grid.Row="1"
Grid.ColumnSpan="1" />
</Grid>
<Label Padding="10,0,0,0" Margin="10,0,0,3" Style="{StaticResource fadingLabelSeperatorStlye}" Grid.Row="1">
Editor
</Label>
<local:SymbologyEditorControl x:Name="editor" Grid.Row="2"/>
</Grid>
</ScrollViewer>
</Border>
and the effect I am observing is illustrated below
How do I fix this?

Answer 1

There was a common WPF bug that caused this behavior of any Popup type UI displaying in back instead of topmost so that may be what you're seeing. I haven't seen the issue in a while but I'm not sure if it was fixed or not. It's related to bad video drivers so you might see it only on certain machines and not all the time.
There was a hotfix available from MS support but it might be easier to just switch your application to use software rendering instead.