UserControl не заполняет ширину ItemsPresenter в настроенном DataGrid

Question

Я пытаюсь добавить настраиваемый заголовок над ячейками DataGrid. Я хочу, чтобы заголовок прокручивался вместе с содержимым DataGrid, поэтому я не хочу настраивать заголовки столбцов DataGrid, потому что они находятся за пределами ScrollViewer. Идея состоит в том, чтобы создать внешний вид веб-страницы (например, Google Classroom), где у вас есть заголовок страницы вверху, содержащий некоторую информацию о классе, и поток сообщений от учителей и учеников внизу.

Я взял стандартный шаблон элемента управления для DataGrid (созданный Visual Studio) и разместил сетку вокруг ItemsPresenter. Первая строка этой сетки содержит мой настраиваемый заголовок (во вложенной сетке), а вторая строка содержит исходный ItemsPresenter. Проблема в том, что ItemsPresenter заполняет доступную ширину, но отображаемые им «элементы» (пользовательский элемент управления с именем AnnouncementView) не заполняют ширину ItemsPresenter. Итак, как вы можете видеть на рисунке ниже, вы не можете ничего прочитать в представленных AnnouncementView.

Стиль DataGrid

<Color x:Key="ClassroomHeaderColor">#FF2222FF</Color>
<SolidColorBrush x:Key="ClassroomHeaderBrush" Color="{StaticResource ClassroomHeaderColor}" />

<Style x:Key="AnnouncementsDataGrid" TargetType="{x:Type DataGrid}">
    <Setter Property="Background" Value="{DynamicResource {x:Static SystemColors.ControlBrushKey}}"/>
    <Setter Property="Foreground" Value="{DynamicResource {x:Static SystemColors.ControlTextBrushKey}}"/>
    <Setter Property="BorderBrush" Value="#FF688CAF"/>
    <Setter Property="BorderThickness" Value="1"/>
    <Setter Property="MaxWidth" Value="1000" />
    <Setter Property="HorizontalAlignment" Value="Stretch" />
    <Setter Property="RowDetailsVisibilityMode" Value="VisibleWhenSelected"/>
    <Setter Property="ScrollViewer.CanContentScroll" Value="true"/>
    <Setter Property="ScrollViewer.PanningMode" Value="Both"/>
    <Setter Property="Stylus.IsFlicksEnabled" Value="False"/>
    <Setter Property="Template">
        <Setter.Value>
            <ControlTemplate TargetType="{x:Type DataGrid}">
                <Border BorderBrush="{TemplateBinding BorderBrush}" BorderThickness="{TemplateBinding BorderThickness}" Background="{TemplateBinding Background}" Padding="{TemplateBinding Padding}" SnapsToDevicePixels="True">
                    <ScrollViewer x:Name="DG_ScrollViewer" Focusable="false">
                        <ScrollViewer.Template>
                            <ControlTemplate TargetType="{x:Type ScrollViewer}">
                                <Grid>
                                    <Grid.ColumnDefinitions>
                                        <ColumnDefinition Width="Auto"/>
                                        <ColumnDefinition Width="*"/>
                                        <ColumnDefinition Width="Auto"/>
                                    </Grid.ColumnDefinitions>
                                    <Grid.RowDefinitions>
                                        <RowDefinition Height="Auto"/>
                                        <RowDefinition Height="*"/>
                                        <RowDefinition Height="Auto"/>
                                    </Grid.RowDefinitions>
                                    <Button Command="{x:Static DataGrid.SelectAllCommand}" Focusable="false" Style="{DynamicResource {ComponentResourceKey ResourceId=DataGridSelectAllButtonStyle, TypeInTargetAssembly={x:Type DataGrid}}}" Visibility="{Binding HeadersVisibility, ConverterParameter={x:Static DataGridHeadersVisibility.All}, Converter={x:Static DataGrid.HeadersVisibilityConverter}, RelativeSource={RelativeSource AncestorType={x:Type DataGrid}}}" Width="{Binding CellsPanelHorizontalOffset, RelativeSource={RelativeSource AncestorType={x:Type DataGrid}}}"/>
                                    <DataGridColumnHeadersPresenter x:Name="PART_ColumnHeadersPresenter" Grid.Column="1" Visibility="{Binding HeadersVisibility, ConverterParameter={x:Static DataGridHeadersVisibility.Column}, Converter={x:Static DataGrid.HeadersVisibilityConverter}, RelativeSource={RelativeSource AncestorType={x:Type DataGrid}}}"/>
                                    <ScrollContentPresenter x:Name="PART_ScrollContentPresenter" CanContentScroll="{TemplateBinding CanContentScroll}" Grid.ColumnSpan="2" Grid.Row="1"/>
                                    <ScrollBar x:Name="PART_VerticalScrollBar" Grid.Column="2" Maximum="{TemplateBinding ScrollableHeight}" Orientation="Vertical" Grid.Row="1" Visibility="{TemplateBinding ComputedVerticalScrollBarVisibility}" Value="{Binding VerticalOffset, Mode=OneWay, RelativeSource={RelativeSource TemplatedParent}}" ViewportSize="{TemplateBinding ViewportHeight}"/>
                                    <Grid Grid.Column="1" Grid.Row="2">
                                        <Grid.ColumnDefinitions>
                                            <ColumnDefinition Width="{Binding NonFrozenColumnsViewportHorizontalOffset, RelativeSource={RelativeSource AncestorType={x:Type DataGrid}}}"/>
                                            <ColumnDefinition Width="*"/>
                                        </Grid.ColumnDefinitions>
                                        <ScrollBar x:Name="PART_HorizontalScrollBar" Grid.Column="1" Maximum="{TemplateBinding ScrollableWidth}" Orientation="Horizontal" Visibility="{TemplateBinding ComputedHorizontalScrollBarVisibility}" Value="{Binding HorizontalOffset, Mode=OneWay, RelativeSource={RelativeSource TemplatedParent}}" ViewportSize="{TemplateBinding ViewportWidth}"/>
                                    </Grid>
                                </Grid>
                            </ControlTemplate>
                        </ScrollViewer.Template>
<!--#region Custom Modification -->
                        <Grid>
                            <Grid.RowDefinitions>
                                <RowDefinition Height="Auto" />
                                <RowDefinition Height="1*" />
                            </Grid.RowDefinitions>
                            <Border Background="{StaticResource ClassroomHeaderBrush}" CornerRadius="8" Grid.Row="0">
                                <Grid>
                                    <Grid.ColumnDefinitions>
                                        <ColumnDefinition Width="Auto" />
                                        <ColumnDefinition Width="1*" />
                                        <ColumnDefinition Width="Auto" />
                                    </Grid.ColumnDefinitions>
                                    <Grid.RowDefinitions>
                                        <RowDefinition MinHeight="25" Height="Auto" />
                                        <RowDefinition MinHeight="20"  Height="Auto" />
                                        <RowDefinition Height="Auto" />
                                        <RowDefinition Height="Auto" />
                                        <RowDefinition Height="Auto" />
                                        <RowDefinition Height="Auto" />
                                        <RowDefinition Height="16" />
                                        <RowDefinition Height="Auto" />
                                    </Grid.RowDefinitions>
                                    <Label Content="{Binding CMCourse.Course.Name}" Grid.Row="0" Grid.Column="0" Grid.ColumnSpan="3" />
                                    <Label Content="{Binding CMCourse.Course.Section}" Grid.Row="1" Grid.Column="0" Grid.ColumnSpan="3" />
                                    <Label Content="Class code" Grid.Row="2" Grid.Column="0"/>
                                    <Label Content="{Binding CMCourse.Course.EnrollmentCode}" Grid.Row="2" Grid.Column="1" />

                                    <!-- Creation and Update times -->
                                    <Label Content="Created:" Grid.Row="2" Grid.Column="1" HorizontalAlignment="Right" />
                                    <Label Content="{Binding CMCourse.Course.CreationTime, StringFormat=g}" Grid.Row="2" Grid.Column="2" />
                                    <Label Content="Last Updated:" Grid.Row="3" Grid.Column="1" HorizontalAlignment="Right" />
                                    <Label Content="{Binding CMCourse.Course.UpdateTime, StringFormat=g}" Grid.Row="3" Grid.Column="2" />

                                    <!-- Course State -->
                                    <Label Content="Course State:" Grid.Row="4" Grid.Column="1" HorizontalAlignment="Right" />
                                    <StackPanel Orientation="Horizontal" Grid.Row="4" Grid.Column="2">
                                        <ComboBox x:Name="NewCourseState"
                                                    IsReadOnly="True"
                                                    ItemsSource="{Binding NewCourseState}"
                                                    SelectedValuePath="Name"
                                                    SelectedItem="{Binding SelectedNewCourseState, Mode=TwoWay}"
                                                    DisplayMemberPath="Name">
                                            <ComboBox.ItemContainerStyle>
                                                <Style TargetType="ComboBoxItem">
                                                    <Setter Property="IsSelected" Value="{Binding IsSelected}" />
                                                </Style>
                                            </ComboBox.ItemContainerStyle>
                                        </ComboBox>
                                        <Button x:Name="ChangeCourseState" Content="Apply"/>
                                    </StackPanel>

                                    <!-- More Details Expander -->
                                    <Expander Header="Details" Grid.Row="5" Grid.Column="0" Grid.ColumnSpan="2">
                                        <Grid>
                                            <Grid.ColumnDefinitions>
                                                <ColumnDefinition Width="Auto" />
                                                <ColumnDefinition Width="1*" />
                                            </Grid.ColumnDefinitions>
                                            <Grid.RowDefinitions>
                                                <RowDefinition Height="Auto" />
                                                <RowDefinition Height="Auto" />
                                                <RowDefinition Height="Auto" />
                                            </Grid.RowDefinitions>
                                            <Label Content="Subject" Grid.Row="0" Grid.Column="0" />
                                            <Label Content="{Binding CMCourse.Course.Subject, TargetNullValue='No subject set'}" Grid.Row="0" Grid.Column="1" />
                                            <Label Content="Room" Grid.Row="1" Grid.Column="0" />
                                            <Label Content="{Binding CMCourse.Course.Room, TargetNullValue='No room set'}" Grid.Row="1" Grid.Column="1" />
                                            <Label Content="{Binding CMCourse.Course.Description, TargetNullValue='No description set'}" Grid.Row="2" Grid.Column="0" Grid.ColumnSpan="2" />
                                        </Grid>
                                    </Expander>
                                </Grid>
                            </Border>

                            <ItemsPresenter SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}" Grid.Row="1" />
                        </Grid>
<!--#endregion Custom Modification-->
                    </ScrollViewer>
                </Border>
            </ControlTemplate>
        </Setter.Value>
    </Setter>
    <Style.Triggers>
        <MultiTrigger>
            <MultiTrigger.Conditions>
                <Condition Property="IsGrouping" Value="true"/>
                <Condition Property="VirtualizingPanel.IsVirtualizingWhenGrouping" Value="false"/>
            </MultiTrigger.Conditions>
            <Setter Property="ScrollViewer.CanContentScroll" Value="false"/>
        </MultiTrigger>
    </Style.Triggers>
</Style>

AnnouncementView - Пользовательский элемент управления, отображаемый DataGrid

<UserControl x:Class="ClassroomManager.Views.AnnouncementView"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" 
             xmlns:d="http://schemas.microsoft.com/expression/blend/2008" 
             xmlns:local="clr-namespace:ClassroomManager.Views"
             mc:Ignorable="d" 
             d:DesignHeight="200" d:DesignWidth="400">
    <Grid>
        <Grid.ColumnDefinitions>
            <ColumnDefinition Width="Auto" />
            <ColumnDefinition Width="1*" />
            <ColumnDefinition Width="Auto" />
        </Grid.ColumnDefinitions>
        <Grid.RowDefinitions>
            <RowDefinition Height="Auto" />
            <RowDefinition Height="Auto" />
            <RowDefinition Height="1*" />
            <RowDefinition Height="Auto" />
        </Grid.RowDefinitions>
    
        <!-- Row 0 -->
        <Image x:Name="ProfilePic" Source="{Binding OwnerProfilePicURL}" Grid.Row="0" Grid.Column="0" Grid.RowSpan="2" />
        <Label x:Name="OwnersName" Content="{Binding OwnerName}" Grid.Row="0" Grid.Column="1" />
        <Label Content="State" Grid.Row="0" Grid.Column="2" />

        <!-- Row 1-->
        <Label x:Name="CreationTime" Content="{Binding CreationTimeText}" Grid.Row="1" Grid.Column="1" />
        <Label x:Name="PostState" Content="{Binding StateText}" Grid.Row="1" Grid.Column="2" />

        <!-- Row 2 -->
        <TextBlock x:Name="AnnouncementText" Text="{Binding Announcement.Text}" Grid.Row="2" Grid.Column="0" Grid.ColumnSpan="3" />

    </Grid>
</UserControl>