Add a filter box to the Debug Steps pane

The step tree can run to hundreds of entries per decompile, so finding a
specific mutation means scrolling. Add a filter box in the pane's top-right
corner: a row survives when its description -- or any descendant's -- contains
the text (case-insensitive), keeping the path to every match, and the tree
auto-expands while filtering so matches nested under transform groups stay
visible. Implemented as an item-visibility converter over the existing TreeView
rather than switching to SharpTreeView, which would change the Steps contract
and rewrite the pane's tests for no functional gain here.

Assisted-by: Claude:claude-opus-4-8:Claude Code

ILSpy.Tests/Views/DebugStepsTests.cs

@@ -19,11 +19,13 @@
 #if DEBUG
 
 using System;
+using System.Globalization;
 using System.Linq;
 using System.Threading.Tasks;
 
 using Avalonia.Controls;
 using Avalonia.Headless.NUnit;
+using Avalonia.Threading;
 using Avalonia.VisualTree;
 
 using AwesomeAssertions;
@@ -332,6 +334,53 @@ public class DebugStepsTests
 		return Task.CompletedTask;
 	}
 
+	[AvaloniaTest]
+	public Task DebugStepFilter_Keeps_Matches_And_The_Path_To_Them()
+	{
+		var converter = new DebugStepFilterConverter();
+		var matchingLeaf = new ICSharpCode.Decompiler.IL.Transforms.Stepper.Node("3: Introduce query continuation");
+		var otherLeaf = new ICSharpCode.Decompiler.IL.Transforms.Stepper.Node("4: Flatten switch section block");
+		var group = new ICSharpCode.Decompiler.IL.Transforms.Stepper.Node("CombineQueryExpressions");
+		group.Children.Add(matchingLeaf);
+		group.Children.Add(otherLeaf);
+
+		// An empty filter shows every row.
+		Filter(group, "").Should().BeTrue();
+		Filter(otherLeaf, "  ").Should().BeTrue();
+		// A group survives because a descendant matches, keeping the path to the match.
+		Filter(group, "continuation").Should().BeTrue();
+		// The matching leaf survives, case-insensitively.
+		Filter(matchingLeaf, "CONTINUATION").Should().BeTrue();
+		// A sibling that neither matches nor leads to a match is hidden.
+		Filter(otherLeaf, "continuation").Should().BeFalse();
+		return Task.CompletedTask;
+
+		bool Filter(ICSharpCode.Decompiler.IL.Transforms.Stepper.Node node, string filter)
+			=> (bool)converter.Convert(new object?[] { node, filter }, typeof(bool), null, CultureInfo.InvariantCulture);
+	}
+
+	[AvaloniaTest]
+	public Task DebugSteps_View_Loads_With_Filter_Applied()
+	{
+		// Guards the filter wiring in the XAML -- a MultiBinding inside a TreeViewItem style Setter
+		// plus the RelativeSource lookups -- against a structural break that x:CompileBindings="False"
+		// would not catch at build time. Realising the view with a populated tree and a live filter
+		// must not throw.
+		var vm = new DebugStepsPaneModel();
+		var group = new ICSharpCode.Decompiler.IL.Transforms.Stepper.Node("CombineQueryExpressions");
+		group.Children.Add(new ICSharpCode.Decompiler.IL.Transforms.Stepper.Node("3: Introduce query continuation"));
+		group.Children.Add(new ICSharpCode.Decompiler.IL.Transforms.Stepper.Node("4: Flatten switch section block"));
+		vm.Steps = new[] { group };
+		vm.IsAvailable = true;
+
+		var window = new Window { Width = 400, Height = 300, Content = new DebugSteps { DataContext = vm } };
+		window.Show();
+		vm.FilterText = "continuation";
+		Dispatcher.UIThread.RunJobs();
+		window.Close();
+		return Task.CompletedTask;
+	}
+
 	[AvaloniaTest]
 	public Task MarkNodeStart_Excludes_Leading_Indentation()
 	{

ILSpy/ViewModels/DebugStepsPaneModel.cs

@@ -100,6 +100,20 @@ namespace ICSharpCode.ILSpy.ViewModels
 		[ObservableProperty]
 		bool isAvailable;
 
+		/// <summary>
+		/// Free-text filter for the step tree; empty shows everything. Bound to the filter box in the
+		/// pane's top-right corner. A row survives when its description -- or a descendant's -- matches.
+		/// </summary>
+		[ObservableProperty]
+		[NotifyPropertyChangedFor(nameof(IsFiltering))]
+		string? filterText;
+
+		/// <summary>
+		/// True while <see cref="FilterText"/> is non-empty. Drives auto-expansion of the tree so that
+		/// matches nested under transform groups are revealed rather than hidden in collapsed groups.
+		/// </summary>
+		public bool IsFiltering => !string.IsNullOrWhiteSpace(FilterText);
+
 		public IRelayCommand ShowStateBeforeCommand { get; }
 		public IRelayCommand ShowStateAfterCommand { get; }
 		public IRelayCommand DebugStepCommand { get; }

ILSpy/Views/DebugStepFilterConverter.cs

@@ -0,0 +1,60 @@
+// Copyright (c) 2026 AlphaSierraPapa for the SharpDevelop Team
+//
+// 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.
+
+#if DEBUG
+
+using System;
+using System.Collections.Generic;
+using System.Globalization;
+
+using Avalonia.Data.Converters;
+
+using ICSharpCode.Decompiler.IL.Transforms;
+
+namespace ICSharpCode.ILSpy.Views
+{
+	/// <summary>
+	/// Decides whether a Debug Steps tree row stays visible under the pane's filter box. A step is
+	/// shown when the filter is empty, or when its description -- or that of any descendant --
+	/// contains the filter text, so the path to every match is preserved. Bound per row against
+	/// [ the step, the filter text ].
+	/// </summary>
+	public sealed class DebugStepFilterConverter : IMultiValueConverter
+	{
+		public object Convert(IList<object?> values, Type targetType, object? parameter, CultureInfo culture)
+		{
+			if (values.Count < 2 || values[1] is not string filter || string.IsNullOrWhiteSpace(filter))
+				return true;
+			return values[0] is Stepper.Node node && Matches(node, filter.Trim());
+		}
+
+		static bool Matches(Stepper.Node node, string filter)
+		{
+			if (node.Description.Contains(filter, StringComparison.OrdinalIgnoreCase))
+				return true;
+			foreach (var child in node.Children)
+			{
+				if (Matches(child, filter))
+					return true;
+			}
+			return false;
+		}
+	}
+}
+
+#endif

ILSpy/Views/DebugSteps.axaml

@@ -3,40 +3,64 @@
              xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
              xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
              xmlns:vm="using:ICSharpCode.ILSpy.ViewModels"
+             xmlns:views="using:ICSharpCode.ILSpy.Views"
              xmlns:il="using:ICSharpCode.Decompiler.IL"
              mc:Ignorable="d" d:DesignWidth="400" d:DesignHeight="300"
              x:Class="ICSharpCode.ILSpy.Views.DebugSteps"
              x:DataType="vm:DebugStepsPaneModel">
+	<UserControl.Resources>
+		<views:DebugStepFilterConverter x:Key="StepFilter" />
+	</UserControl.Resources>
 	<Panel>
 	<TextBlock IsVisible="{Binding !IsAvailable}"
 	           HorizontalAlignment="Center" VerticalAlignment="Center"
 	           Margin="8" TextWrapping="Wrap" Opacity="0.7"
 	           Text="Debug steps are not available for the current language." />
 	<DockPanel IsVisible="{Binding IsAvailable}">
-		<!-- Options are contributed by the active step-provider language and selected by type:
-		     ILAst renders its writing-options checkboxes; C# (null Options) renders nothing. -->
-		<ContentControl DockPanel.Dock="Top" Margin="2" Content="{Binding Options}">
-			<ContentControl.DataTemplates>
-				<DataTemplate DataType="il:ILAstWritingOptions">
-					<StackPanel Orientation="Horizontal">
-						<CheckBox Margin="3" Content="Field sugar"
-						          IsChecked="{Binding UseFieldSugar, Mode=TwoWay}" />
-						<CheckBox Margin="3" Content="Logic operation sugar"
-						          IsChecked="{Binding UseLogicOperationSugar, Mode=TwoWay}" />
-						<CheckBox Margin="3" Content="Show IL ranges"
-						          IsChecked="{Binding ShowILRanges, Mode=TwoWay}" />
-						<CheckBox Margin="3" Content="Show child index in block"
-						          IsChecked="{Binding ShowChildIndexInBlock, Mode=TwoWay}" />
-					</StackPanel>
-				</DataTemplate>
-			</ContentControl.DataTemplates>
-		</ContentControl>
+		<!-- Top row: language-contributed options on the left, filter box in the top-right corner.
+		     Options are selected by type: ILAst renders its writing-options checkboxes; C# (null
+		     Options) renders nothing, leaving just the filter box. -->
+		<DockPanel DockPanel.Dock="Top" Margin="2">
+			<TextBox DockPanel.Dock="Right" Width="160" Margin="4,0,0,0"
+			         VerticalAlignment="Center" PlaceholderText="Filter steps"
+			         Text="{Binding FilterText}" />
+			<ContentControl Content="{Binding Options}">
+				<ContentControl.DataTemplates>
+					<DataTemplate DataType="il:ILAstWritingOptions">
+						<StackPanel Orientation="Horizontal">
+							<CheckBox Margin="3" Content="Field sugar"
+							          IsChecked="{Binding UseFieldSugar, Mode=TwoWay}" />
+							<CheckBox Margin="3" Content="Logic operation sugar"
+							          IsChecked="{Binding UseLogicOperationSugar, Mode=TwoWay}" />
+							<CheckBox Margin="3" Content="Show IL ranges"
+							          IsChecked="{Binding ShowILRanges, Mode=TwoWay}" />
+							<CheckBox Margin="3" Content="Show child index in block"
+							          IsChecked="{Binding ShowChildIndexInBlock, Mode=TwoWay}" />
+						</StackPanel>
+					</DataTemplate>
+				</ContentControl.DataTemplates>
+			</ContentControl>
+		</DockPanel>
 		<TreeView Name="StepsTree"
 		          ItemsSource="{Binding Steps}"
 		          SelectedItem="{Binding SelectedStep, Mode=TwoWay}"
 		          DoubleTapped="OnTreeDoubleTapped"
 		          KeyDown="OnTreeKeyDown"
 		          x:CompileBindings="False">
+			<TreeView.Styles>
+				<!-- Hide rows that neither match the filter nor lead to a match, and expand the tree
+				     while filtering so surviving matches under transform groups stay visible. -->
+				<Style Selector="TreeViewItem">
+					<Setter Property="IsVisible">
+						<MultiBinding Converter="{StaticResource StepFilter}">
+							<Binding />
+							<Binding Path="DataContext.FilterText" RelativeSource="{RelativeSource AncestorType=TreeView}" />
+						</MultiBinding>
+					</Setter>
+					<Setter Property="IsExpanded"
+					        Value="{Binding DataContext.IsFiltering, RelativeSource={RelativeSource AncestorType=TreeView}}" />
+				</Style>
+			</TreeView.Styles>
 			<TreeView.ItemTemplate>
 				<TreeDataTemplate ItemsSource="{Binding Children}">
 					<TextBlock Text="{Binding Description}" />