why aren't the buttons aligning horizontally in the stackpanel?

Question

I have a list of buttons being generated from data, so there are a variable number of buttons. In an old version of the software I am overhauling, they used random custom controls but the result was that there was an infinitely growing horizontal scroller.

It appears I have the exact same XAML but it's not aligning each item horizontally, only vertically

<ScrollViewer Background="#33FFFFFF" HorizontalScrollBarVisibility="Visible" VerticalScrollBarVisibility="Disabled" >
    <StackPanel Orientation="Horizontal">
        <ItemsControl ItemsSource="{Binding Events}">
            <ItemsControl.ItemTemplate>
                <DataTemplate>
                    <Button Margin="10" HorizontalAlignment="Stretch" Background="#FF1B1B1B" BorderThickness="5" BorderBrush="White" Command="{Binding Source={StaticResource Locator}, Path=EventSelector.ViewEventCommand}" CommandParameter="{Binding }">
                        <Grid Name="tileGridButton" Height="600" Width="400" Margin="5">
                            <Grid.RowDefinitions>
                                <RowDefinition Height="5*"></RowDefinition>
                                <RowDefinition Height="5*"></RowDefinition>
                            </Grid.RowDefinitions>
                            <Border Grid.Row="0" Name="tileImageBorder" Margin="5" BorderThickness="1" VerticalAlignment="Stretch" HorizontalAlignment="Center">
                                <Image Name="tileImage" Margin="0" Source="{Binding ImageURL}"/>
                            </Border>
                            <Grid Grid.Row="1" VerticalAlignment="Top">
                                <Grid.RowDefinitions>
                                    <RowDefinition Height="6*"></RowDefinition>
                                    <RowDefinition Height="4*"></RowDefinition>
                                </Grid.RowDefinitions>
                                <TextBlock Grid.Row="0" Name="tileText" Margin="5" Foreground="White" TextAlignment="Center" TextTrimming="CharacterEllipsis" TextWrapping="Wrap" FontSize="30" FontWeight="Bold" Text="{Binding Title}" />
                                <TextBlock Grid.Row="1" Name="tileDescription" Margin="5" Foreground="White" TextAlignment="Center" TextTrimming="CharacterEllipsis" TextWrapping="Wrap" FontSize="25" FontWeight="Bold" Text="{Binding EventTimeBegin}" />
                            </Grid>
                        </Grid>
                    </Button>
                </DataTemplate>
            </ItemsControl.ItemTemplate>
        </ItemsControl>
    </StackPanel>
</ScrollViewer>

Answer 1

First of all your StackPanel is pretty much useless because it only contains a single item, the ItemsControl.

Instead you need to change your ItemsControl so that it uses a StackPanel as its method of laying out items inside the ItemsControl...

<ItemsControl ...>
    <ItemsControl.ItemsPanel>
        <ItemsPanelTemplate>
            <StackPanel Orientation="Horizontal"/>
        <ItemsPanelTemplate>
    </ItemsControl.ItemsPanel>
    <ItemsControl.ItemTemplate>
    .....