WPF Scrollviewer不会向上滚动

<Grid>
    <Grid.ColumnDefinitions>
        <ColumnDefinition Width=\"143*\" />
        <ColumnDefinition Width=\"135*\" />
    </Grid.ColumnDefinitions>
    <TabControl Margin=\"12,29\" Name=\"tabControl1\" Grid.ColumnSpan=\"2\">
        <TabItem Header=\"TabItem1\" Name=\"tabItem1\">
            <ScrollViewer Height=\"440\" Name=\"scrollViewer1\" Width=\"872\" HorizontalAlignment=\"Left\" VerticalAlignment=\"Top\">
                <Grid Height=\"440\" Name=\"grid1\" Width=\"851\" VerticalAlignment=\"Top\" HorizontalAlignment=\"Left\">
...