Use separate FilterSettings for each tab and bind the dropdowns directly to the settings of the active tab page. This should fix #2404.

ILSpy/Analyzers/AnalyzerTreeView.cs

@@ -35,6 +35,8 @@ namespace ICSharpCode.ILSpy.Analyzers
 	/// </summary>
 	public class AnalyzerTreeView : SharpTreeView
 	{
+		FilterSettings filterSettings;
+
 		public AnalyzerTreeView()
 		{
 			this.ShowRoot = false;
@@ -42,7 +44,21 @@ namespace ICSharpCode.ILSpy.Analyzers
 			this.BorderThickness = new Thickness(0);
 			ContextMenuProvider.Add(this);
 			MainWindow.Instance.CurrentAssemblyListChanged += MainWindow_Instance_CurrentAssemblyListChanged;
-			MainWindow.Instance.SessionSettings.FilterSettings.PropertyChanged += FilterSettings_PropertyChanged;
+			DockWorkspace.Instance.PropertyChanged += DockWorkspace_PropertyChanged;
+			filterSettings = MainWindow.Instance.SessionSettings.FilterSettings;
+			filterSettings.PropertyChanged += FilterSettings_PropertyChanged;
+		}
+
+		private void DockWorkspace_PropertyChanged(object sender, System.ComponentModel.PropertyChangedEventArgs e)
+		{
+			switch (e.PropertyName)
+			{
+				case nameof(DockWorkspace.Instance.ActiveTabPage):
+					filterSettings.PropertyChanged -= FilterSettings_PropertyChanged;
+					filterSettings = DockWorkspace.Instance.ActiveTabPage.FilterSettings;
+					filterSettings.PropertyChanged += FilterSettings_PropertyChanged;
+					break;
+			}
 		}
 
 		private void FilterSettings_PropertyChanged(object sender, System.ComponentModel.PropertyChangedEventArgs e)

ILSpy/Docking/DockWorkspace.cs

@@ -94,8 +94,6 @@ namespace ICSharpCode.ILSpy.Docking
 				if (_activeTabPage != value)
 				{
 					_activeTabPage = value;
-					this.sessionSettings.FilterSettings.Language = value.Language;
-					this.sessionSettings.FilterSettings.LanguageVersion = value.LanguageVersion;
 					var state = value.GetState();
 					if (state != null)
 					{
@@ -173,23 +171,6 @@ namespace ICSharpCode.ILSpy.Docking
 		internal void LoadSettings(SessionSettings sessionSettings)
 		{
 			this.sessionSettings = sessionSettings;
-			sessionSettings.FilterSettings.PropertyChanged += FilterSettings_PropertyChanged;
-		}
-
-		private void FilterSettings_PropertyChanged(object sender, PropertyChangedEventArgs e)
-		{
-			if (e.PropertyName == "Language")
-			{
-				ActiveTabPage.Language = sessionSettings.FilterSettings.Language;
-				if (sessionSettings.FilterSettings.Language.HasLanguageVersions)
-				{
-					sessionSettings.FilterSettings.LanguageVersion = ActiveTabPage.LanguageVersion;
-				}
-			}
-			else if (e.PropertyName == "LanguageVersion")
-			{
-				ActiveTabPage.LanguageVersion = sessionSettings.FilterSettings.LanguageVersion;
-			}
 		}
 
 		internal void CloseAllTabs()

ILSpy/FilterSettings.cs

@@ -17,6 +17,7 @@
 // DEALINGS IN THE SOFTWARE.
 
 using System;
+using System.Collections.Generic;
 using System.ComponentModel;
 using System.Linq;
 using System.Runtime.CompilerServices;
@@ -34,6 +35,13 @@ namespace ICSharpCode.ILSpy
 	/// </remarks>
 	public class FilterSettings : INotifyPropertyChanged
 	{
+		/// <summary>
+		/// This dictionary is necessary to remember language versions across language changes. For example, 
+		/// the user first select C# 10, then switches to IL, then switches back to C#. After that we must be
+		/// able to restore the original selection (i.e., C# 10).
+		/// </summary>
+		private readonly Dictionary<Language, LanguageVersion> languageVersionHistory = new Dictionary<Language, LanguageVersion>();
+
 		public FilterSettings(XElement element)
 		{
 			this.ShowApiLevel = (ApiVisibility?)(int?)element.Element("ShowAPILevel") ?? ApiVisibility.PublicAndInternal;
@@ -146,8 +154,27 @@ namespace ICSharpCode.ILSpy
 			set {
 				if (language != value)
 				{
+					if (language != null && language.HasLanguageVersions)
+					{
+						languageVersionHistory[language] = languageVersion;
+					}
 					language = value;
 					OnPropertyChanged();
+					if (language.HasLanguageVersions)
+					{
+						if (languageVersionHistory.TryGetValue(value, out var version))
+						{
+							LanguageVersion = version;
+						}
+						else
+						{
+							LanguageVersion = Language.LanguageVersions.Last();
+						}
+					}
+					else
+					{
+						LanguageVersion = default;
+					}
 				}
 			}
 		}
@@ -167,6 +194,10 @@ namespace ICSharpCode.ILSpy
 				if (languageVersion != value)
 				{
 					languageVersion = value;
+					if (language.HasLanguageVersions)
+					{
+						languageVersionHistory[language] = languageVersion;
+					}
 					OnPropertyChanged();
 				}
 			}

ILSpy/MainWindow.xaml

@@ -19,7 +19,7 @@
 	xmlns:styles="urn:TomsToolbox.Wpf.Styles"
 	xmlns:b="http://schemas.microsoft.com/xaml/behaviors"
 	xmlns:themes="clr-namespace:ICSharpCode.ILSpy.Themes"
-	d:DataContext="{d:DesignInstance local:MainWindowDataContext}"
+	d:DataContext="{d:DesignInstance local:MainWindowViewModel}"
 >
 	<Window.Resources>
 		<controls:BoolToVisibilityConverter x:Key="BooleanToVisibilityConverter" />
@@ -103,9 +103,9 @@
 			<MenuItem Header="{x:Static properties:Resources._File}" />
 			<!-- contents of file menu are added using MEF -->
 			<MenuItem Header="{x:Static properties:Resources._View}">
-				<MenuItem Header="{x:Static properties:Resources.Show_publiconlyTypesMembers}" IsCheckable="True" IsChecked="{Binding SessionSettings.FilterSettings.ApiVisPublicOnly}" />
-				<MenuItem Header="{x:Static properties:Resources.Show_internalTypesMembers}" IsCheckable="True" IsChecked="{Binding SessionSettings.FilterSettings.ApiVisPublicAndInternal}" />
-				<MenuItem Header="{x:Static properties:Resources.Show_allTypesAndMembers}" IsCheckable="True" IsChecked="{Binding SessionSettings.FilterSettings.ApiVisAll}" />
+				<MenuItem Header="{x:Static properties:Resources.Show_publiconlyTypesMembers}" IsCheckable="True" IsChecked="{Binding Workspace.ActiveTabPage.FilterSettings.ApiVisPublicOnly}" />
+				<MenuItem Header="{x:Static properties:Resources.Show_internalTypesMembers}" IsCheckable="True" IsChecked="{Binding Workspace.ActiveTabPage.FilterSettings.ApiVisPublicAndInternal}" />
+				<MenuItem Header="{x:Static properties:Resources.Show_allTypesAndMembers}" IsCheckable="True" IsChecked="{Binding Workspace.ActiveTabPage.FilterSettings.ApiVisAll}" />
 				<Separator/>
 				<MenuItem Header="{x:Static properties:Resources.UILanguage}">
 					<MenuItem Header="{x:Static properties:Resources.UILanguage_System}" IsCheckable="True" IsChecked="{Binding SessionSettings.CurrentCulture, Converter={StaticResource cultureSelectionConverter}, ConverterParameter={x:Null}}" />
@@ -156,25 +156,25 @@
 				<Image Width="16" Height="16" Source="{controls:XamlResource Images/AssemblyList}" Style="{StaticResource DarkModeAwareImageStyle}"/>
 			</Button>
 			<Separator />
-			<CheckBox IsChecked="{Binding SessionSettings.FilterSettings.ApiVisPublicOnly}" ToolTip="{x:Static properties:Resources.ShowPublicOnlyTypesMembers}">
+			<CheckBox IsChecked="{Binding Workspace.ActiveTabPage.FilterSettings.ApiVisPublicOnly}" ToolTip="{x:Static properties:Resources.ShowPublicOnlyTypesMembers}">
 				<Image Width="16" Height="16" Source="{controls:XamlResource Images/ShowPublicOnly}" Style="{StaticResource DarkModeAwareImageStyle}"/>
 			</CheckBox>
-			<CheckBox IsChecked="{Binding SessionSettings.FilterSettings.ApiVisPublicAndInternal}" ToolTip="{x:Static properties:Resources.ShowInternalTypesMembers}">
+			<CheckBox IsChecked="{Binding Workspace.ActiveTabPage.FilterSettings.ApiVisPublicAndInternal}" ToolTip="{x:Static properties:Resources.ShowInternalTypesMembers}">
 				<Image Width="16" Height="16" Source="{controls:XamlResource Images/ShowPrivateInternal}" Style="{StaticResource DarkModeAwareImageStyle}"/>
 			</CheckBox>
-			<CheckBox IsChecked="{Binding SessionSettings.FilterSettings.ApiVisAll}" ToolTip="{x:Static properties:Resources.ShowAllTypesAndMembers}">
+			<CheckBox IsChecked="{Binding Workspace.ActiveTabPage.FilterSettings.ApiVisAll}" ToolTip="{x:Static properties:Resources.ShowAllTypesAndMembers}">
 				<Image Width="16" Height="16" Source="{controls:XamlResource Images/ShowAll}" Style="{StaticResource DarkModeAwareImageStyle}"/>
 			</CheckBox>
 			<Separator />
 			<ComboBox Name="languageComboBox" DisplayMemberPath="Name" Width="100" MaxDropDownHeight="Auto"
-					  IsEnabled="{Binding ActiveTabPage.SupportsLanguageSwitching, Source={x:Static docking:DockWorkspace.Instance}}"
+					  IsEnabled="{Binding Workspace.ActiveTabPage.SupportsLanguageSwitching}"
 			          ItemsSource="{x:Static local:Languages.AllLanguages}" ToolTip="{x:Static properties:Resources.SelectLanguageDropdownTooltip}"
-			          SelectedItem="{Binding SessionSettings.FilterSettings.Language}"/>
+			          SelectedItem="{Binding Workspace.ActiveTabPage.FilterSettings.Language}"/>
 			<ComboBox Name="languageVersionComboBox" DisplayMemberPath="DisplayName" Width="120" MaxDropDownHeight="Auto" ToolTip="{x:Static properties:Resources.SelectVersionDropdownTooltip}"
 			          Visibility="{Binding SelectedItem.HasLanguageVersions, ElementName=languageComboBox, Converter={StaticResource BooleanToVisibilityConverter}}"
-					  IsEnabled="{Binding ActiveTabPage.SupportsLanguageSwitching, Source={x:Static docking:DockWorkspace.Instance}}"
+					  IsEnabled="{Binding Workspace.ActiveTabPage.SupportsLanguageSwitching}"
 			          ItemsSource="{Binding SelectedItem.LanguageVersions, ElementName=languageComboBox, UpdateSourceTrigger=PropertyChanged}"
-			          SelectedItem="{Binding SessionSettings.FilterSettings.LanguageVersion, UpdateSourceTrigger=PropertyChanged}"/>
+			          SelectedItem="{Binding Workspace.ActiveTabPage.FilterSettings.LanguageVersion, UpdateSourceTrigger=PropertyChanged}"/>
 			<Separator />
 			<CheckBox IsChecked="{Binding SessionSettings.IsDarkMode}" ToolTip="{x:Static properties:Resources.DarkMode}">
 				<Image Source="Images/DarkMode.png" Stretch="None" Style="{StaticResource DarkModeAwareImageStyle}"/>

ILSpy/MainWindow.xaml.cs

@@ -32,7 +32,6 @@ using System.Windows.Controls;
 using System.Windows.Input;
 using System.Windows.Interop;
 using System.Windows.Media;
-using System.Windows.Media.Imaging;
 using System.Windows.Navigation;
 using System.Windows.Threading;
 
@@ -55,13 +54,6 @@ using Microsoft.Win32;
 
 namespace ICSharpCode.ILSpy
 {
-	class MainWindowDataContext
-	{
-		public DockWorkspace Workspace { get; set; }
-		public SessionSettings SessionSettings { get; set; }
-		public AssemblyListManager AssemblyListManager { get; set; }
-	}
-
 	/// <summary>
 	/// The main window of the application.
 	/// </summary>
@@ -70,7 +62,8 @@ namespace ICSharpCode.ILSpy
 		bool refreshInProgress, changingActiveTab;
 		readonly NavigationHistory<NavigationState> history = new NavigationHistory<NavigationState>();
 		ILSpySettings spySettingsForMainWindow_Loaded;
-		internal SessionSettings sessionSettings;
+		SessionSettings sessionSettings;
+		FilterSettings filterSettings;
 		AssemblyList assemblyList;
 		AssemblyListTreeNode assemblyListTreeNode;
 
@@ -115,7 +108,7 @@ namespace ICSharpCode.ILSpy
 			// Make sure Images are initialized on the UI thread.
 			this.Icon = Images.ILSpyIcon;
 
-			this.DataContext = new MainWindowDataContext {
+			this.DataContext = new MainWindowViewModel {
 				Workspace = DockWorkspace.Instance,
 				SessionSettings = sessionSettings,
 				AssemblyListManager = AssemblyListManager
@@ -130,8 +123,10 @@ namespace ICSharpCode.ILSpy
 			InitializeComponent();
 			InitToolPanes();
 			DockWorkspace.Instance.InitializeLayout(DockManager);
-			sessionSettings.FilterSettings.PropertyChanged += filterSettings_PropertyChanged;
 			sessionSettings.PropertyChanged += SessionSettings_PropertyChanged;
+			filterSettings = sessionSettings.FilterSettings;
+			filterSettings.PropertyChanged += filterSettings_PropertyChanged;
+			DockWorkspace.Instance.PropertyChanged += DockWorkspace_PropertyChanged;
 			InitMainMenu();
 			InitToolbar();
 			ContextMenuProvider.Add(AssemblyTreeView);
@@ -139,6 +134,18 @@ namespace ICSharpCode.ILSpy
 			this.Loaded += MainWindow_Loaded;
 		}
 
+		private void DockWorkspace_PropertyChanged(object sender, PropertyChangedEventArgs e)
+		{
+			switch (e.PropertyName)
+			{
+				case nameof(DockWorkspace.Instance.ActiveTabPage):
+					filterSettings.PropertyChanged -= filterSettings_PropertyChanged;
+					filterSettings = DockWorkspace.Instance.ActiveTabPage.FilterSettings;
+					filterSettings.PropertyChanged += filterSettings_PropertyChanged;
+					break;
+			}
+		}
+
 		private void SessionSettings_PropertyChanged(object sender, PropertyChangedEventArgs e)
 		{
 			switch (e.PropertyName)
@@ -406,7 +413,7 @@ namespace ICSharpCode.ILSpy
 		{
 			LoadAssemblies(args.AssembliesToLoad, commandLineLoadedAssemblies, focusNode: false);
 			if (args.Language != null)
-				sessionSettings.FilterSettings.Language = Languages.GetLanguage(args.Language);
+				filterSettings.Language = Languages.GetLanguage(args.Language);
 			return true;
 		}
 
@@ -597,8 +604,7 @@ namespace ICSharpCode.ILSpy
 		void MainWindow_Loaded(object sender, RoutedEventArgs e)
 		{
 			DockWorkspace.Instance.TabPages.Add(new TabPageModel() {
-				Language = CurrentLanguage,
-				LanguageVersion = CurrentLanguageVersion
+				FilterSettings = filterSettings.Clone()
 			});
 			DockWorkspace.Instance.ActiveTabPage = DockWorkspace.Instance.TabPages.First();
 
@@ -770,7 +776,7 @@ namespace ICSharpCode.ILSpy
 			assemblyList.CollectionChanged += assemblyList_Assemblies_CollectionChanged;
 
 			assemblyListTreeNode = new AssemblyListTreeNode(assemblyList);
-			assemblyListTreeNode.FilterSettings = sessionSettings.FilterSettings.Clone();
+			assemblyListTreeNode.FilterSettings = filterSettings.Clone();
 			assemblyListTreeNode.Select = x => SelectNode(x, inNewTabPage: false);
 			AssemblyTreeView.Root = assemblyListTreeNode;
 
@@ -837,7 +843,7 @@ namespace ICSharpCode.ILSpy
 			// Thus, the main window will use one mutable instance (for data-binding), and assign a new clone to the ILSpyTreeNodes whenever the main
 			// mutable instance changes.
 			if (assemblyListTreeNode != null)
-				assemblyListTreeNode.FilterSettings = sessionSettings.FilterSettings.Clone();
+				assemblyListTreeNode.FilterSettings = DockWorkspace.Instance.ActiveTabPage.FilterSettings.Clone();
 		}
 
 		internal AssemblyListTreeNode AssemblyListTreeNode {
@@ -866,8 +872,7 @@ namespace ICSharpCode.ILSpy
 					{
 						DockWorkspace.Instance.TabPages.Add(
 							new TabPageModel() {
-								Language = CurrentLanguage,
-								LanguageVersion = CurrentLanguageVersion
+								FilterSettings = filterSettings.Clone()
 							});
 						DockWorkspace.Instance.ActiveTabPage = DockWorkspace.Instance.TabPages.Last();
 						AssemblyTreeView.SelectedItem = null;
@@ -913,8 +918,7 @@ namespace ICSharpCode.ILSpy
 			{
 				DockWorkspace.Instance.TabPages.Add(
 					new TabPageModel() {
-						Language = CurrentLanguage,
-						LanguageVersion = CurrentLanguageVersion
+						FilterSettings = filterSettings.Clone()
 					});
 				DockWorkspace.Instance.ActiveTabPage = DockWorkspace.Instance.TabPages.Last();
 			}
@@ -1200,7 +1204,10 @@ namespace ICSharpCode.ILSpy
 			{
 				state = DockWorkspace.Instance.ActiveTabPage.GetState() as DecompilerTextViewState;
 			}
-			DecompileSelectedNodes(state);
+			if (!changingActiveTab)
+			{
+				DecompileSelectedNodes(state);
+			}
 
 			SelectionChanged?.Invoke(sender, e);
 		}
@@ -1268,8 +1275,10 @@ namespace ICSharpCode.ILSpy
 			}
 		}
 
-		public Language CurrentLanguage => sessionSettings.FilterSettings.Language;
-		public LanguageVersion CurrentLanguageVersion => sessionSettings.FilterSettings.LanguageVersion;
+		public Language CurrentLanguage => DockWorkspace.Instance.ActiveTabPage.FilterSettings.Language;
+		public LanguageVersion CurrentLanguageVersion => DockWorkspace.Instance.ActiveTabPage.FilterSettings.LanguageVersion;
+
+		public bool SupportsLanguageSwitching => DockWorkspace.Instance.ActiveTabPage.SupportsLanguageSwitching;
 
 		public event SelectionChangedEventHandler SelectionChanged;
 
@@ -1341,8 +1350,7 @@ namespace ICSharpCode.ILSpy
 				{
 					DockWorkspace.Instance.TabPages.Add(
 						new TabPageModel() {
-							Language = CurrentLanguage,
-							LanguageVersion = CurrentLanguageVersion
+							FilterSettings = filterSettings.Clone()
 						});
 					DockWorkspace.Instance.ActiveTabPage = DockWorkspace.Instance.TabPages.Last();
 				}
@@ -1407,6 +1415,7 @@ namespace ICSharpCode.ILSpy
 			sessionSettings.ActiveAutoLoadedAssembly = GetAutoLoadedAssemblyNode(AssemblyTreeView.SelectedItem as SharpTreeNode);
 			sessionSettings.WindowBounds = this.RestoreBounds;
 			sessionSettings.DockLayout.Serialize(new XmlLayoutSerializer(DockManager));
+			sessionSettings.FilterSettings = DockWorkspace.Instance.ActiveTabPage.FilterSettings.Clone();
 			sessionSettings.Save();
 		}
 

ILSpy/MainWindowViewModel.cs

@@ -0,0 +1,30 @@
+// Copyright (c) 2021 Siegfried Pammer
+// 
+// Permission is hereby granted, free of charge, to any person obtaining a copy of this
+// software and associated documentation files (the "Software"), to deal in the Software
+// without restriction, including without limitation the rights to use, copy, modify, merge,
+// publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons
+// to whom the Software is furnished to do so, subject to the following conditions:
+// 
+// The above copyright notice and this permission notice shall be included in all copies or
+// substantial portions of the Software.
+// 
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED,
+// INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR
+// PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE
+// FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR
+// OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
+// DEALINGS IN THE SOFTWARE.
+
+using ICSharpCode.ILSpy.Docking;
+using ICSharpCode.ILSpy.ViewModels;
+
+namespace ICSharpCode.ILSpy
+{
+	class MainWindowViewModel
+	{
+		public DockWorkspace Workspace { get; set; }
+		public SessionSettings SessionSettings { get; set; }
+		public AssemblyListManager AssemblyListManager { get; set; }
+	}
+}

ILSpy/Search/SearchPane.cs

@@ -50,6 +50,7 @@ namespace ICSharpCode.ILSpy
 		RunningSearch currentSearch;
 		bool runSearchOnNextShow;
 		IComparer<SearchResult> resultsComparer;
+		FilterSettings filterSettings;
 
 		public static readonly DependencyProperty ResultsProperty =
 			DependencyProperty.Register("Results", typeof(ObservableCollection<SearchResult>), typeof(SearchPane),
@@ -76,13 +77,27 @@ namespace ICSharpCode.ILSpy
 
 			ContextMenuProvider.Add(listBox);
 			MainWindow.Instance.CurrentAssemblyListChanged += MainWindow_Instance_CurrentAssemblyListChanged;
-			MainWindow.Instance.SessionSettings.FilterSettings.PropertyChanged += FilterSettings_PropertyChanged;
+			DockWorkspace.Instance.PropertyChanged += DockWorkspace_PropertyChanged;
+			filterSettings = MainWindow.Instance.SessionSettings.FilterSettings;
+			filterSettings.PropertyChanged += FilterSettings_PropertyChanged;
 			CompositionTarget.Rendering += UpdateResults;
 
 			// This starts empty search right away, so do at the end (we're still in ctor)
 			searchModeComboBox.SelectedIndex = (int)MainWindow.Instance.SessionSettings.SelectedSearchMode;
 		}
 
+		private void DockWorkspace_PropertyChanged(object sender, PropertyChangedEventArgs e)
+		{
+			switch (e.PropertyName)
+			{
+				case nameof(DockWorkspace.Instance.ActiveTabPage):
+					filterSettings.PropertyChanged -= FilterSettings_PropertyChanged;
+					filterSettings = DockWorkspace.Instance.ActiveTabPage.FilterSettings;
+					filterSettings.PropertyChanged += FilterSettings_PropertyChanged;
+					break;
+			}
+		}
+
 		void MainWindow_Instance_CurrentAssemblyListChanged(object sender, NotifyCollectionChangedEventArgs e)
 		{
 			if (IsVisible)

ILSpy/SessionSettings.cs

@@ -76,7 +76,7 @@ namespace ICSharpCode.ILSpy
 			PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
 		}
 
-		public FilterSettings FilterSettings { get; private set; }
+		public FilterSettings FilterSettings { get; internal set; }
 		public SearchMode SelectedSearchMode { get; set; }
 
 		public bool IsDarkMode {

ILSpy/ViewModels/ManageAssemblyListsViewModel.cs

@@ -191,9 +191,9 @@ namespace ICSharpCode.ILSpy.ViewModels
 			if (manager.AssemblyLists.Count > 0)
 			{
 				SelectedAssemblyList = manager.AssemblyLists[Math.Max(0, index - 1)];
-				if (MainWindow.Instance.sessionSettings.ActiveAssemblyList == assemblyList)
+				if (MainWindow.Instance.SessionSettings.ActiveAssemblyList == assemblyList)
 				{
-					MainWindow.Instance.sessionSettings.ActiveAssemblyList = SelectedAssemblyList;
+					MainWindow.Instance.SessionSettings.ActiveAssemblyList = SelectedAssemblyList;
 				}
 			}
 		}
@@ -234,9 +234,9 @@ namespace ICSharpCode.ILSpy.ViewModels
 				string assemblyList = SelectedAssemblyList;
 				SelectedAssemblyList = dlg.ListName;
 				manager.RenameList(assemblyList, dlg.ListName);
-				if (MainWindow.Instance.sessionSettings.ActiveAssemblyList == assemblyList)
+				if (MainWindow.Instance.SessionSettings.ActiveAssemblyList == assemblyList)
 				{
-					MainWindow.Instance.sessionSettings.ActiveAssemblyList = manager.AssemblyLists[manager.AssemblyLists.Count - 1];
+					MainWindow.Instance.SessionSettings.ActiveAssemblyList = manager.AssemblyLists[manager.AssemblyLists.Count - 1];
 				}
 			}
 		}
@@ -372,7 +372,7 @@ namespace ICSharpCode.ILSpy.ViewModels
 
 		private void ExecuteSelectAssemblyList()
 		{
-			MainWindow.Instance.sessionSettings.ActiveAssemblyList = SelectedAssemblyList;
+			MainWindow.Instance.SessionSettings.ActiveAssemblyList = SelectedAssemblyList;
 			this.parent.Close();
 		}
 	}

ILSpy/ViewModels/TabPageModel.cs

@@ -27,61 +27,36 @@ namespace ICSharpCode.ILSpy.ViewModels
 {
 	public class TabPageModel : PaneModel
 	{
-		private readonly Dictionary<Language, LanguageVersion> languageVersionHistory = new Dictionary<Language, LanguageVersion>();
-
 		public TabPageModel()
 		{
 			this.Title = Properties.Resources.NewTab;
 		}
 
-		private Language language;
-		public Language Language {
-			get => language;
+		private FilterSettings filterSettings;
+
+		public FilterSettings FilterSettings {
+			get => filterSettings;
 			set {
-				if (language != value)
+				if (filterSettings != value)
 				{
-					if (language != null && language.HasLanguageVersions)
-					{
-						languageVersionHistory[language] = languageVersion;
-					}
-					language = value;
-					RaisePropertyChanged(nameof(Language));
-					if (language.HasLanguageVersions)
-					{
-						if (languageVersionHistory.TryGetValue(value, out var version))
-						{
-							LanguageVersion = version;
-						}
-						else
-						{
-							LanguageVersion = Language.LanguageVersions.Last();
-						}
-					}
-					else
-					{
-						LanguageVersion = default;
-					}
+					filterSettings = value;
+					RaisePropertyChanged(nameof(FilterSettings));
 				}
 			}
 		}
 
-		private LanguageVersion languageVersion;
+		public Language Language {
+			get => filterSettings.Language;
+			set => filterSettings.Language = value;
+		}
+
 		public LanguageVersion LanguageVersion {
-			get => languageVersion;
-			set {
-				if (languageVersion != value)
-				{
-					languageVersion = value;
-					if (language.HasLanguageVersions)
-					{
-						languageVersionHistory[language] = languageVersion;
-					}
-					RaisePropertyChanged(nameof(LanguageVersion));
-				}
-			}
+			get => filterSettings.LanguageVersion;
+			set => filterSettings.LanguageVersion = value;
 		}
 
 		private bool supportsLanguageSwitching = true;
+
 		public bool SupportsLanguageSwitching {
 			get => supportsLanguageSwitching;
 			set {
@@ -94,6 +69,7 @@ namespace ICSharpCode.ILSpy.ViewModels
 		}
 
 		private object content;
+
 		public object Content {
 			get => content;
 			set {

ILSpy/Views/DebugSteps.xaml.cs

@@ -1,4 +1,5 @@
 using System;
+using System.ComponentModel;
 using System.Windows;
 using System.Windows.Controls;
 using System.Windows.Input;
@@ -22,13 +23,16 @@ namespace ICSharpCode.ILSpy
 #if DEBUG
 		ILAstLanguage language;
 #endif
+		FilterSettings filterSettings;
 
 		public DebugSteps()
 		{
 			InitializeComponent();
 
 #if DEBUG
-			MainWindow.Instance.SessionSettings.FilterSettings.PropertyChanged += FilterSettings_PropertyChanged;
+			DockWorkspace.Instance.PropertyChanged += DockWorkspace_PropertyChanged;
+			this.filterSettings = MainWindow.Instance.SessionSettings.FilterSettings;
+			filterSettings.PropertyChanged += FilterSettings_PropertyChanged;
 			MainWindow.Instance.SelectionChanged += SelectionChanged;
 			writingOptions.PropertyChanged += WritingOptions_PropertyChanged;
 
@@ -41,6 +45,18 @@ namespace ICSharpCode.ILSpy
 #endif
 		}
 
+		private void DockWorkspace_PropertyChanged(object sender, PropertyChangedEventArgs e)
+		{
+			switch (e.PropertyName)
+			{
+				case nameof(DockWorkspace.Instance.ActiveTabPage):
+					filterSettings.PropertyChanged -= FilterSettings_PropertyChanged;
+					filterSettings = DockWorkspace.Instance.ActiveTabPage.FilterSettings;
+					filterSettings.PropertyChanged += FilterSettings_PropertyChanged;
+					break;
+			}
+		}
+
 		private void WritingOptions_PropertyChanged(object sender, System.ComponentModel.PropertyChangedEventArgs e)
 		{
 			DecompileAsync(lastSelectedStep);