WPF

<Style TargetType="{x:Type local:MaroisDropDownTree}">

<pre>
<Setter Property="Template">

    <Setter.Value>

        <ControlTemplate TargetType="{x:Type ComboBox}">

            <Border Margin="2" 
                    Background="{TemplateBinding Background}" 
                    BorderBrush="{TemplateBinding BorderBrush}" 
                    BorderThickness="{TemplateBinding BorderThickness}" 
                    CornerRadius="{TemplateBinding BorderThickness}">

                <TreeView ItemsSource="{Binding TreeData, RelativeSource ={RelativeSource TemplatedParent}}"/>

            </Border>

        </ControlTemplate>

    </Setter.Value>

</Setter>

CommandManager.InvalidateRequerySuggested();

<Button Grid.Row="7"
        Grid.Column="0"
        Content="Sign In"
        Command="{Binding SignInCommand}"
        Margin="0,0,0,0"/>

using CommunityToolkit.Mvvm.Input;
using Marois.Framework.Core.Shared;
using System.Windows.Input;

namespace Jayhawk.UI.WPF.ViewModels
{
    public class LoginViewModel : _DialogViewModelBase
    {
        #region Private Fields
        private readonly IEventAggregator _eventAggregator;
        #endregion

        #region Properties
        private string? _UserName;
        public string? UserName
        {
            get { return _UserName; }
            set
            {
                SetProperty(nameof(UserName), ref _UserName, value);
            }
        }

        private string? _Password;
        public string? Password
        {
            get { return _Password; }
            set
            {
                SetProperty(nameof(Password), ref _Password, value);
            }
        }

        private bool _IsRememberMeChecked;
        public bool IsRememberMeChecked
        {
            get { return _IsRememberMeChecked; }
            set
            {
                SetProperty(nameof(IsRememberMeChecked), ref _IsRememberMeChecked, value);
            }
        }

        private bool _IsLoggingIn;
        public bool IsLoggingIn
        {
            get { return _IsLoggingIn; }
            set
            {
                SetProperty(nameof(IsLoggingIn), ref _IsLoggingIn, value);
            }
        }
        #endregion

        #region Commands
        private ICommand? _SignInCommand;
        public ICommand? SignInCommand
        {
            get
            {
                if (_SignInCommand == null)
                    _SignInCommand = new RelayCommand(SignInExecuted, SignInCanExecute);
                return _SignInCommand;
            }
        }
        #endregion

        #region CTOR
        public LoginViewModel()
        {
        }
        #endregion

        #region Private Methods
        private bool SignInCanExecute()
        {
            return !string.IsNullOrEmpty(UserName) && !string.IsNullOrEmpty(Password);
        }

        private void SignInExecuted()
        {
            IsLoggingIn = true;

            // TODO: Do the log in
        }
        #endregion
    }
}

<ListBox Grid.Row="2"
            Grid.Column="1"
            x:Name="outerList"
            Margin="2,2,2,2"
            Background="Transparent"
            ItemsSource="{Binding RecentItemSections}"
            SelectedItem="{Binding SelectedRecentItemSection}"
            HorizontalAlignment="Stretch"
            BorderBrush="Yellow"
            BorderThickness="2">

    <ListBox.ItemTemplate>

        <DataTemplate>

            <Expander IsExpanded="{Binding IsSectionExpanded}"
                        x:Name="expander"
                        HorizontalAlignment="Stretch"
                        BorderBrush="Red"
                        BorderThickness="2">

                <Expander.Header>
                    <TextBlock Text="{Binding SectionName}"
                                Foreground="White" 
                                Width="{Binding RelativeSource={RelativeSource Mode=FindAncestor,
                                    AncestorType={x:Type ListBoxItem}}, Path=ActualWidth}"/>
                </Expander.Header>

                <ListBox ItemsSource="{Binding RecentItems}"
                            BorderBrush="Green"
                            BorderThickness="5"
                            Margin="2,0,20,0">

                    <ListBox.ItemTemplate>
                        <DataTemplate>

                            <Border BorderBrush="MediumAquamarine"
                                    BorderThickness="2">

                                <StackPanel Orientation="Vertical"
                                        HorizontalAlignment="Stretch">

                                    <TextBlock Text="{Binding ItemName}"
                                            FontSize="12"
                                            Margin="2"/>

                                    <TextBlock Text="{Binding ItemLocation}"
                                            FontSize="10"
                                            Margin="2,0,2,2"/>

                                </StackPanel>

                            </Border>

                        </DataTemplate>
                    </ListBox.ItemTemplate>

                </ListBox>

            </Expander>

        </DataTemplate>

    </ListBox.ItemTemplate>

</ListBox>