Das Problem, das ich habe, ist die Größe der gruppierten Rasteransichten aller Gruppen auf die Größe derzuerst Gruppe wie im Screenshot unten:

Ich brauche Gruppen mit unterschiedlichen Breiten, um ihre Kinder unterzubringen. In diesem Fall sollte die zweite Gruppe breiter und die dritte Gruppe schmaler sein.

Der Code für die Test-App, die ich geschrieben habe, ist wie folgt:

XAML (MainPage.xaml)

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

<Grid Margin="100" Background="{StaticResource ApplicationPageBackgroundThemeBrush}">
    <GridView x:Name="GroupingGridView"
              ItemsSource="{Binding Source={StaticResource GroupedCollectionViewSource}}"
              SelectionMode="None">
        <GridView.ItemsPanel>
            <ItemsPanelTemplate>
                <VariableSizedWrapGrid Margin="20"
                                       Background="MidnightBlue"
                                       Orientation="Horizontal" />
            </ItemsPanelTemplate>
        </GridView.ItemsPanel>
        <GridView.GroupStyle>
            <GroupStyle>
                <GroupStyle.HeaderTemplate>
                    <DataTemplate>
                        <TextBlock FontSize="24"
                                   Foreground="White"
                                   Text="{Binding Key}" />
                    </DataTemplate>
                </GroupStyle.HeaderTemplate>

                <GroupStyle.Panel>
                    <ItemsPanelTemplate>
                        <VariableSizedWrapGrid Margin="20"
                                               Background="CornflowerBlue"
                                               Orientation="Horizontal" />
                    </ItemsPanelTemplate>
                </GroupStyle.Panel>
            </GroupStyle>
        </GridView.GroupStyle>
    </GridView>
</Grid>

Code dahinter (MainPage.xaml.cs)

using System.Collections.Generic;
using Windows.UI.Xaml.Controls;
using Windows.UI.Xaml.Navigation;

namespace GroupingBugTest
{
    /// <summary>
    /// An empty page that can be used on its own or navigated to within a Frame.
    /// </summary>
    public sealed partial class MainPage : Page
{
    public MainPage()
    {
        this.InitializeComponent();
    }

    /// <summary>
    /// Invoked when this page is about to be displayed in a Frame.
    /// </summary>
    /// <param name="e">Event data that describes how this page was reached.  The Parameter
    /// property is typically used to configure the page.</param>
    protected override void OnNavigatedTo(NavigationEventArgs e)
    {
        PopulateGroupedCollectionViewSource();
    }

    private void PopulateGroupedCollectionViewSource()
    {
        List<GroupInfoList<string>> groupedCollection = new List<GroupInfoList<string>>();

        var firstGroup = new GroupInfoList<string>() { Key = "FIRST GROUP (5 items)" };
        var secondGroup = new GroupInfoList<string>() { Key = "SECOND GROUP (10 items)" };
        var thirdGroup = new GroupInfoList<string>() { Key = "THIRD GROUP (2 items)" };

        for (int i = 1; i <= 10; i++)
        {
            if (i <= 5) //add 5 items to first group
            {
                firstGroup.Add(i.ToString());
            }

            secondGroup.Add(i.ToString()); //add 10 items to second group

            if (i <= 2) //add 2 items to third group
            {
                thirdGroup.Add(i.ToString());
            }
        }

        groupedCollection.Add(firstGroup);
        groupedCollection.Add(secondGroup);
        groupedCollection.Add(thirdGroup);

        GroupedCollectionViewSource.Source = groupedCollection;
    }
}

//Taken from Microsoft Windows 8 code samples
public class GroupInfoList<T> : List<object>
{
    public object Key { get; set; }

    public new IEnumerator<object> GetEnumerator()
    {
        return (System.Collections.Generic.IEnumerator<object>)base.GetEnumerator();
    }
}
}