TextBlock not wrapping in UserControl as screen width decreases

Question

I'm trying to get my TextBlock inside a UserControl to wrap when the windows is snapped to one side/screen width is smaller but for some it won't wrap properly and keeps stretching over the screen limits, even though I set the horizontal alignment of the UserControl to the left. Does anyone know why this problem is happening and how to fix this?

<UserControl
    x:Class="MyApp.UserControls.MyUserControl"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:local="using:MyApp.UserControls"
    xmlns:controls="using:Microsoft.Toolkit.Uwp.UI.Controls"
    xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
    mc:Ignorable="d"
    HorizontalAlignment="Left">

    <controls:DropShadowPanel x:Name="grd">
        <Grid>
            <Grid.RowDefinitions>
                <RowDefinition Height="Auto"/>
                <RowDefinition Height="Auto"/>
            </Grid.RowDefinitions>

            <StackPanel Grid.Row="0" Orientation="Horizontal">
                <TextBlock x:Name="txtItem" TextWrapping="Wrap"/>
            </StackPanel>
            <StackPanel Grid.Row="1" Orientation="Vertical">
                <TextBlock x:Name="txtTitle"/>
                <TextBlock x:Name="txtSubtitle"/>
            </StackPanel>
        </Grid>
    </controls:DropShadowPanel>
</UserControl>

MainPage

<Page
    x:Class="MyApp.MainPage"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:local="using:MyApp.MainPage"
    xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
    mc:Ignorable="d">

    <Grid xmlns:src="using:MyApp.UserControls">
        <Grid.RowDefinitions>
            <RowDefinition Height="Auto"/>
            <RowDefinition Height="*"/>
        </Grid.RowDefinitions>
        <StackPanel Grid.Row="0">
            <TextBlock x:Name="txtTitle" />
            <TextBlock x:Name="txtSubtitle""/>
        </StackPanel>
        <ScrollViewer Grid.Row="1">
            <Grid>
                <Grid.RowDefinitions>
                    <RowDefinition Height="Auto"/>
                </Grid.RowDefinitions>
                <Grid.ColumnDefinitions>
                    <ColumnDefinition Width="Auto"/>
                </Grid.ColumnDefinitions>

                <StackPanel x:Name="myStackPanel" HorizontalAlignment="Left"/>
            </Grid>
        </ScrollViewer>
    </Grid>
</Page>

Answer 1

ColumnDefinition at line 25 in MainPage must be *

<Page
     x:Class="MyApp.MainPage"
     xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
     xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
     xmlns:local="using:MyApp.MainPage"
     xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
     xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
     mc:Ignorable="d">

    <Grid>
        <Grid.RowDefinitions>
            <RowDefinition Height="Auto"/>
            <RowDefinition Height="*"/>
        </Grid.RowDefinitions>
        <StackPanel Grid.Row="0">
            <TextBlock x:Name="txtTitle" />
            <TextBlock x:Name="txtSubtitle"/>
        </StackPanel>
        <ScrollViewer Grid.Row="1">
            <Grid>
                <Grid.RowDefinitions>
                    <RowDefinition Height="Auto"/>
                </Grid.RowDefinitions>
                <Grid.ColumnDefinitions>
                    <ColumnDefinition Width="*"/>
                </Grid.ColumnDefinitions>

                <StackPanel x:Name="myStackPanel" HorizontalAlignment="Left"/>
            </Grid>
        </ScrollViewer>
    </Grid>
 </Page>

Orientation of StackPanel at line 12 must be Vertical

<UserControl [...]>
     <controls:DropShadowPanel x:Name="grd">
         <Grid>
             <Grid.RowDefinitions>
                 <RowDefinition Height="Auto"/>
                 <RowDefinition Height="Auto"/>
             </Grid.RowDefinitions>

             <StackPanel Grid.Row="0" Orientation="Vertical">
                 <TextBlock x:Name="txtItem" TextWrapping="Wrap"/>
             </StackPanel>
             <StackPanel Grid.Row="1" Orientation="Vertical">
                 <TextBlock x:Name="txtTitle"/>
                 <TextBlock x:Name="txtSubtitle"/>
             </StackPanel>
         </Grid>
     </controls:DropShadowPanel>
 </UserControl>