タイトルパネルを追加すると、スクロールできなくなりました。回復するのを手伝ってください。
<StackPanel>
<Grid HorizontalAlignment="Left">
<Grid.RowDefinitions>
<RowDefinition Height="Auto"/>
<RowDefinition Height="Auto"/>
</Grid.RowDefinitions>
<TextBlock .../>
<TextBlock ... >
</TextBlock>
</Grid>
<ScrollViewer Grid.Row="1" x:Name="_ScrollViewer">
<StackPanel ScrollViewer.VerticalScrollBarVisibility="Disabled">
<Grid ScrollViewer.VerticalScrollBarVisibility="Disabled">
<Grid.RowDefinitions>
<RowDefinition Height="auto" />
<RowDefinition Height="auto" />
</Grid.RowDefinitions>
<ListBox x:Name="listmy"
Grid.Row="0" ScrollViewer.VerticalScrollBarVisibility="Disabled"
>
<ListBox.ItemTemplate>
<DataTemplate>
<local:TypeMessage Content="{Binding}">
<local:TypeMessage.Me>
<DataTemplate>
...
</DataTemplate>
</local:TypeMessage.Me>
<local:TypeMessage.Other>
<DataTemplate>
...
</DataTemplate>
</local:TypeMessage.Other>
</local:TypeMessage>
</DataTemplate>
</ListBox.ItemTemplate>
<ListBox.ItemsPanel>
<ItemsPanelTemplate>
<VirtualizingStackPanel />
</ItemsPanelTemplate>
</ListBox.ItemsPanel>
</ListBox>
<Grid Grid.Row="3" ScrollViewer.VerticalScrollBarVisibility="Disabled">
<Grid.RowDefinitions>
<RowDefinition Height="auto" />
<RowDefinition Height="auto" />
</Grid.RowDefinitions>
<tbox:WatermarkTextBox
ScrollViewer.VerticalScrollBarVisibility="Disabled"
/>
</Grid>
</Grid>
</StackPanel>
</ScrollViewer>
</StackPanel>
</phone:PhoneApplicationPage>