UWP 应用部署在平板电脑上，UserControl 控件里面的ComboBox控件展开时被UserControl遮挡，PC端不存在此问题

Question

    <Grid x:Name="grdRoot">
        <Grid.RowDefinitions>
            <RowDefinition Height="40" />
            <RowDefinition Height="auto" />
            <RowDefinition Height="auto" />
            <RowDefinition Height="auto" />

        </Grid.RowDefinitions>

        <TextBlock Margin="15,0,0,0" VerticalAlignment="Center">色</TextBlock>

        <Grid
            x:Name="grdSetColors"
            Grid.Row="1"
            Margin="8,0,8,0">
            <Grid.RowDefinitions>
                <RowDefinition Height="58" />
                <RowDefinition Height="55" />
                <RowDefinition Height="55" />
                <RowDefinition Height="55" />
                <RowDefinition Height="55" />
            </Grid.RowDefinitions>
            <Grid.ColumnDefinitions>
                <ColumnDefinition Width="50" />
                <ColumnDefinition />
                <ColumnDefinition />
                <ColumnDefinition />
                <ColumnDefinition />
                <ColumnDefinition />
            </Grid.ColumnDefinitions>

        </Grid>

        <Grid x:Name="grdSetThickness" Grid.Row="2">
            <Grid.RowDefinitions>
                <RowDefinition />
                <RowDefinition Height="40" />
                <RowDefinition />
            </Grid.RowDefinitions>
            <TextBlock
                Margin="15,10,0,0"
                VerticalAlignment="Center"
                Text="サイズ" />

            <Line
                x:Name="showLine"
                Grid.Row="1"
                VerticalAlignment="Center"
                Stroke="Black"
                StrokeThickness="{Binding ElementName=MySlider, Path=Value, UpdateSourceTrigger=PropertyChanged, Mode=TwoWay}"
                X1="26"
                X2="288"
                Y1="20"
                Y2="20" />
            <Slider
                x:Name="MySlider"
                Grid.Row="2"
                Width="270"
                Margin="20,10,20,0"
                HorizontalAlignment="Left"
                Maximum="24"
                Minimum="1"
                ValueChanged="MySlider_ValueChanged"
                Value="{Binding ElementName=showLine, Path=StrokeThickness}" />

        </Grid>

        <Grid x:Name="grdSetTextStyle" Grid.Row="3">
            <Grid.RowDefinitions>
                <RowDefinition />
                <RowDefinition Height="50" />
                <RowDefinition />
                <RowDefinition />
                <RowDefinition />
            </Grid.RowDefinitions>

            <TextBlock
                Margin="15,10,0,0"
                VerticalAlignment="Center"
                Text="フォント" />

            <ScrollViewer Grid.Row="1">
                <TextBlock
                    x:Name="showFontFamily"
                    Margin="15,10,0,0"
                    HorizontalAlignment="Center"
                    VerticalAlignment="Top"
                    FontFamily="{Binding ElementName=cbo_fontFamily, Path=SelectedItem, UpdateSourceTrigger=PropertyChanged, Mode=OneWay}"
                    FontSize="{Binding ElementName=cbo_fontSize, Path=SelectedItem, UpdateSourceTrigger=PropertyChanged, Mode=OneWay}"
                    Text="Font type" />
            </ScrollViewer>

            <StackPanel Grid.Row="2" IsHitTestVisible="True">
                <ComboBox
                    x:Name="cbo_fontFamily"
                    Width="270"
                    Margin="20,10,0,0"
                    SelectedItem="{Binding ElementName=showFontFamily, Path=FontFamily}"
                    SelectionChanged="cbo_fontFamily_SelectionChanged" />

        

            </StackPanel>
            <TextBlock
                Grid.Row="3"
                Margin="15,20,0,0"
                VerticalAlignment="Center"
                Text="フォントサイズ" />
            <ComboBox
                x:Name="cbo_fontSize"
                Grid.Row="4"
                Width="270"
                Margin="20,10,0,0"
                IsHitTestVisible="True"
                SelectedItem="{Binding ElementName=showFontFamily, Path=FontSize}"
                SelectionChanged="cbo_fontSize_SelectionChanged" />

        </Grid>



    </Grid>


</StackPanel>