It seems to be a generelly problem on most radwindows what we have build in.
Here is a code snipped:
<telerikNavigation:RadWindow x:Name="chartwindow" IsRestricted="False" MaxWidth="1100" MaxHeight="550" telerik:StyleManager.Theme="Windows7" WindowStartupLocation="CenterScreen" ResizeMode="NoResize" Header="{Binding Path=ResTour.radpanel7, Source={StaticResource LocalizedStrings}}">
<Grid x:Name="LayoutRoot8" ShowGridLines="False">
<Grid.Background>
<LinearGradientBrush EndPoint="0.5,1" StartPoint="0.5,0">
<GradientStop Color="#FF8C9ABC" Offset="0"/>
<GradientStop Color="#FF6B7B9F" Offset="1"/>
</LinearGradientBrush>
</Grid.Background>
<Grid.RowDefinitions>
<RowDefinition Height="10" />
<RowDefinition Height="495" />
<RowDefinition Height="10" />
</Grid.RowDefinitions>
<Grid.ColumnDefinitions>
<ColumnDefinition Width="10" />
<ColumnDefinition Width="490" />
<ColumnDefinition Width="490" />
<ColumnDefinition Width="10" />
</Grid.ColumnDefinitions>
<telerikCharting:RadChart x:Name="radchart" Grid.Row="1" Grid.Column="1" Grid.ColumnSpan="2" telerik:StyleManager.Theme="Office_Blue" BorderBrush="#41578B" BorderThickness="1,1,1,1" FontFamily="Verdana" FontSize="11">
<telerikCharting:RadChart.Effect>
<DropShadowEffect BlurRadius="8" ShadowDepth="4"/>
</telerikCharting:RadChart.Effect>
</telerikCharting:RadChart>
</Grid>
</telerikNavigation:RadWindow>