MainWindow.xaml

<?xml version="1.0" encoding="utf-8"?>
<Window
    x:Class="TreeViewItemDropIssue.MainWindow"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:local="using:TreeViewItemDropIssue"
    xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
    mc:Ignorable="d"
    Title="TreeViewItemDropIssue">
    <StackPanel
        Orientation="Horizontal"
        HorizontalAlignment="Left"
        VerticalAlignment="Top"
        Width="500"
        Height="500"
        Background="{ThemeResource ApplicationPageBackgroundThemeBrush}">
        <Grid
            CanDrag="True"
            DragStarting="Grid_DragStarting"
            Height="130"
            Width="130"
            HorizontalAlignment="Left"
            VerticalAlignment="Top"
            Background="LightPink"
            BorderThickness="1"
            BorderBrush="HotPink">
            <TextBlock
                Text="Drag Me!"
                FontSize="22"
                FontWeight="Bold"
                VerticalAlignment="Center"
                HorizontalAlignment="Center"/>
        </Grid>

        <Grid
            VerticalAlignment="Top"
            Margin="20, 0, 0, 0"
            BorderThickness="1"
            BorderBrush="{ThemeResource SystemControlForegroundBaseMediumLowBrush}">
            <Grid.Resources>
                <local:TreeViewTemplateSelector x:Key="TreeViewTemplateSelector">
                    <local:TreeViewTemplateSelector.GroupTemplate>
                        <DataTemplate x:DataType="local:TreeViewTestGroupItem">
                            <TreeViewItem
                                Content="{x:Bind Title}"
                                ItemsSource="{x:Bind MembersItemSource}"
                                AllowDrop="False"
                                CanDrag="False"
                                FontWeight="SemiBold"/>
                        </DataTemplate>
                    </local:TreeViewTemplateSelector.GroupTemplate>

                    <local:TreeViewTemplateSelector.MemberTemplate>
                        <DataTemplate x:DataType="local:TreeViewTestMemberItem">
                            <TreeViewItem
                                Content="{x:Bind Name}"
                                AllowDrop="True"
                                CanDrag="False"
                                DragOver="TreeViewItem_DragOver"
                                DragLeave="TreeViewItem_DragLeave"
                                Drop="TreeViewItem_Drop"
                                Height="50"
                                Width="300"/>
                        </DataTemplate>
                    </local:TreeViewTemplateSelector.MemberTemplate>
                </local:TreeViewTemplateSelector>
            </Grid.Resources>

            <TreeView
                ItemTemplateSelector="{StaticResource TreeViewTemplateSelector}"
                ItemsSource="{x:Bind GroupsItemSource}"
                SelectionMode="None"
                AllowDrop="False"
                CanDragItems="False"
                CanReorderItems="False"
                Width="200">
                <TreeView.ItemContainerStyle>
                    <Style TargetType="TreeViewItem">
                        <Setter Property="IsExpanded" Value="True"/>
                    </Style>
                </TreeView.ItemContainerStyle>
            </TreeView>
        </Grid>
    </StackPanel>
</Window>