htmlplaceholder on a radwindow

[nowell]

Hi:
I placed a htmlpalceholder radwindow in, radwindow high and wide, I set to 800,
I placed a htmlplaceholder on a radwindow,and I set the height and width of radwindow to 800,it shows ok,but when I  scoll
the window to bottom,the htmlplaceholder does not move with the window,and displays strange,the problems can be seen in the attach files
It's my XAML code

 <Grid>

        <Grid.ColumnDefinitions>

            <ColumnDefinition Width="Auto" />

            <ColumnDefinition Width="*"/>

        </Grid.ColumnDefinitions>

        <StackPanel Orientation="Vertical" HorizontalAlignment="Left" Visibility="Visible">

            <TextBlock Text="Select Department"/>

            <tkinput:RadComboBox x:Name="DeptCombx" Width="100" Height="auto" VerticalAlignment="Top"/>

            <Image Source="/A4PM.StationQuery;component/images/pic1.png" Height="77" Width="85"/>

            <TextBlock Text="概述"/>

            <Image Source="/A4PM.StationQuery;component/images/pic1.png" Height="77" Width="85"/>

            <TextBlock Text="工艺流程图"/>

            <Image Source="/A4PM.StationQuery;component/images/pic1.png" Height="77" Width="85"/>

            <TextBlock Text="设备台帐"/>

            <Image Source="/A4PM.StationQuery;component/images/pic1.png" Height="77" Width="85"/>

            <TextBlock Text="HSE信息"/>

            <Image Source="/A4PM.StationQuery;component/images/pic1.png" Height="77" Width="85"/>

            <TextBlock Text="城市简介"/>

        </StackPanel>

        <Grid Grid.Column="1">

                <tkinput:RadHtmlPlaceholder Grid.Column="1" x:Name="htmlholder" HorizontalAlignment="Stretch" VerticalAlignment="Stretch" RespectSilverlightLayoutMeasure="True"/>

        </Grid>

    </Grid>

                                                                                                                                                      thanks