Bug DockingManager not updating state when closing a tab

[PatrickHofman]

In my app, we update the buttons in a ribbon control based upon the state of the last actived document. This works perfectly, until you remove a document. The newly selected document is activated on the UI side, but it is not activated programmatically, causing the bindings to be set according the new situation.

Test code:

WPF MainWindow:

<Window x:Class="WpfApp2.MainWindow"
        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:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:local="clr-namespace:WpfApp2"
        xmlns:xcad="https://github.com/Dirkster99/AvalonDock"
        mc:Ignorable="d"
        d:DataContext="{x:Type local:AppViewModel}"
        Title="MainWindow" Height="450" Width="800">
    <DockPanel>
        <Button Content="Click me" DockPanel.Dock="Top" IsEnabled="{Binding ActiveDocument.IsEnabled}" />

        <xcad:DockingManager DocumentsSource="{Binding Documents, Mode=TwoWay}"
                             AnchorablesSource="{Binding Path=ActiveDocument.ToolPanes}"
                             ActiveContent="{Binding ActivePane, Mode=TwoWay}"
        >
            <DockingManager.DocumentHeaderTemplate>
                <DataTemplate>
                    <Grid>
                        <Grid.ColumnDefinitions>
                            <ColumnDefinition Width="Auto"/>
                            <ColumnDefinition Width="*"/>
                        </Grid.ColumnDefinitions>
                        <TextBlock x:Name="TitleText" Grid.Column="1" Margin="1,0,1,0" MaxWidth="200"
                                   Text="{Binding Content.Title, FallbackValue=#ERROR#}"
                                   ToolTip="{Binding Content.Title}" TextTrimming="CharacterEllipsis"/>
                    </Grid>
                </DataTemplate>
            </DockingManager.DocumentHeaderTemplate>
            
            <DockingManager.LayoutItemTemplate>
                <DataTemplate>
                    <Label Content="{Binding Content.Text}" HorizontalContentAlignment="Center" VerticalContentAlignment="Center" Background="LightBlue" />
                </DataTemplate>
            </DockingManager.LayoutItemTemplate>
        </xcad:DockingManager>
    </DockPanel>
</Window>

Models:

using System.Collections.ObjectModel;
using System.Windows;

namespace WpfApp2
{
    public class AppViewModel : DependencyObject
    {
        public static readonly DependencyProperty DocumentsProperty = DependencyProperty.Register(nameof(Documents), typeof(ObservableCollection<Document>), typeof(AppViewModel));

        public static readonly DependencyProperty ActiveDocumentProperty = DependencyProperty.Register(nameof(ActiveDocument), typeof(Document), typeof(AppViewModel));

        public static readonly DependencyProperty ActivePaneProperty = DependencyProperty.Register(nameof(ActivePane), typeof(ISomeAvalonDockControl), typeof(AppViewModel), new PropertyMetadata(ActivePane_PropertyChanged));

        private static void ActivePane_PropertyChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
        {
            if (d is AppViewModel self)
            {
                if (e.NewValue is Document dvm)
                {
                    self.ActiveDocument = dvm;
                }
                else if (e.NewValue == null)
                {
                    self.ActiveDocument = null;
                }
            }
        }

        public ObservableCollection<Document> Documents
        {
            get { return (ObservableCollection<Document>)this.GetValue(DocumentsProperty); }
            set { this.SetValue(DocumentsProperty, value); }
        }

        public Document ActiveDocument
        {
            get { return (Document)this.GetValue(ActiveDocumentProperty); }
            set { this.SetValue(ActiveDocumentProperty, value); }
        }

        public ISomeAvalonDockControl ActivePane
        {
            get { return (ISomeAvalonDockControl)this.GetValue(ActivePaneProperty); }
            set { this.SetValue(ActivePaneProperty, value); }
        }
    }

    public class Document : DependencyObject, ISomeAvalonDockControl
    {
        public static readonly DependencyProperty TitleProperty = DependencyProperty.Register(nameof(Title), typeof(string), typeof(Document));

        public static readonly DependencyProperty TextProperty = DependencyProperty.Register(nameof(Text), typeof(string), typeof(Document));

        public static readonly DependencyProperty IsEnabledProperty = DependencyProperty.Register(nameof(IsEnabled), typeof(bool), typeof(Document));

        public static readonly DependencyProperty ToolPanesProperty = DependencyProperty.Register(nameof(ToolPanes), typeof(ObservableCollection<ToolPane>), typeof(Document));

        public string Title
        {
            get { return (string)this.GetValue(TitleProperty); }
            set { this.SetValue(TitleProperty, value); }
        }

        public string Text
        {
            get { return (string)this.GetValue(TextProperty); }
            set { this.SetValue(TextProperty, value); }
        }

        public bool IsEnabled
        {
            get { return (bool)this.GetValue(IsEnabledProperty); }
            set { this.SetValue(IsEnabledProperty, value); }
        }

        public ObservableCollection<ToolPane> ToolPanes
        {
            get { return (ObservableCollection<ToolPane>)this.GetValue(ToolPanesProperty); }
            set { this.SetValue(ToolPanesProperty, value); }
        }
    }

    public class ToolPane : DependencyObject, ISomeAvalonDockControl
    {
    }

    public interface ISomeAvalonDockControl
    { }
}

I would expect ActivePane to be updated when a tab is closed. It doesn't and remains null after the it was set to null when closing the previous active document.

To see this visually, do the following:

1. Start the app.
2. Select the second document (it is the first document with true in its name). It should enable the button on top.
3. Close the second document .
4. The UI gets updated: another document is shown, but the button keeps its old state.

I have made a fix already, but you can argue what document should be activated, and if this is the right place to update the state. I will leave that to you.