Let the editor gutter margins react to the left button only

AvaloniaEdit's line-number margin moves the caret and selects the line,
and a fold marker toggles its fold, on any pointer button; the WPF
margins only reacted to the left button. A middle click opens a
reference in a new tab over the text, so one that lands on the gutter
should do nothing instead of folding code or moving the caret. The text
area itself still sees every button.

#4042

Assisted-by: Claude:claude-fable-5-1:Claude Code

ILSpy.Tests/Editor/MarginPointerTests.cs

@@ -0,0 +1,120 @@
+// Copyright (c) 2026 Christoph Wille
+//
+// 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 System.Linq;
+using System.Threading.Tasks;
+
+using Avalonia;
+using Avalonia.Controls;
+using Avalonia.Headless.NUnit;
+using Avalonia.Input;
+using Avalonia.VisualTree;
+
+using AvaloniaEdit.Editing;
+using AvaloniaEdit.Folding;
+
+using AwesomeAssertions;
+
+using ICSharpCode.ILSpy.AppEnv;
+using ICSharpCode.ILSpy.Options;
+using ICSharpCode.ILSpy.TextView;
+using ICSharpCode.ILSpy.TreeNodes;
+
+using NUnit.Framework;
+
+namespace ICSharpCode.ILSpy.Tests.Editor;
+
+/// <summary>
+/// The gutter margins react to the left button only, as they did in WPF: AvaloniaEdit's line-number
+/// margin otherwise selects the line and a fold marker toggles on any button, so a middle press
+/// (which opens references in a new tab over the text) would also move the caret or fold code when
+/// it lands on the gutter.
+/// </summary>
+[TestFixture]
+public class MarginPointerTests
+{
+	static async Task<(ICSharpCode.ILSpy.Views.MainWindow Window, DecompilerTextView View)> SetupAsync()
+	{
+		var (window, vm) = await TestHarness.BootAsync(1);
+		AppComposition.Current.GetExport<SettingsService>().DisplaySettings.ShowLineNumbers = true;
+
+		// A type (not a single method) so the decompiled body carries foldings and the folding margin
+		// is installed.
+		var coreLibName = typeof(object).Assembly.GetName().Name!;
+		var objectNode = vm.AssemblyTreeModel.FindNode<TypeTreeNode>(coreLibName, "System", "System.Object");
+		vm.AssemblyTreeModel.SelectNode(objectNode);
+		await vm.DockWorkspace.WaitForDecompiledTextAsync();
+		var view = await window.WaitForComponent<DecompilerTextView>();
+		await Waiters.WaitForAsync(() => view.Editor.TextArea.LeftMargins.OfType<FoldingMargin>().Any(),
+			description: "the folding margin to be installed for the decompiled type");
+		await Waiters.WaitForIdleAsync();
+		return (window, view);
+	}
+
+	[AvaloniaTest]
+	public async Task Middle_Press_On_The_Line_Number_Margin_Does_Not_Select_The_Line()
+	{
+		// The line-number margin draws text only, which the headless hit test does not see, so the
+		// press is raised on the margin directly; it still tunnels down from the window through the
+		// text area, which is where the margins' button filter sits.
+		var (window, view) = await SetupAsync();
+		var margin = view.Editor.TextArea.LeftMargins.OfType<LineNumberMargin>().Single();
+		var caretBefore = view.Editor.CaretOffset;
+
+		margin.RaiseEvent(MiddlePress(window, margin, new Point(margin.Bounds.Width / 2, margin.Bounds.Height / 4)));
+		await Waiters.WaitForIdleAsync();
+
+		view.Editor.TextArea.Selection.IsEmpty.Should().BeTrue("a middle press on the line numbers must not select the line");
+		view.Editor.CaretOffset.Should().Be(caretBefore, "a middle press on the line numbers must not move the caret");
+
+		margin.RaiseEvent(LeftPress(window, margin, new Point(margin.Bounds.Width / 2, margin.Bounds.Height / 4)));
+		await Waiters.WaitForIdleAsync();
+		view.Editor.CaretOffset.Should().NotBe(caretBefore, "the same press with the left button moves the caret, so the middle press really reached the margin");
+	}
+
+	static PointerPressedEventArgs MiddlePress(Window window, Control target, Avalonia.Point position)
+		=> Press(window, target, position, RawInputModifiers.MiddleMouseButton, PointerUpdateKind.MiddleButtonPressed);
+
+	static PointerPressedEventArgs LeftPress(Window window, Control target, Avalonia.Point position)
+		=> Press(window, target, position, RawInputModifiers.LeftMouseButton, PointerUpdateKind.LeftButtonPressed);
+
+	static PointerPressedEventArgs Press(Window window, Control target, Avalonia.Point position,
+		RawInputModifiers button, PointerUpdateKind kind)
+		=> new(target, new Pointer(Pointer.GetNextFreeId(), PointerType.Mouse, isPrimary: true), window,
+			target.TranslatePoint(position, window)!.Value, 0, new PointerPointProperties(button, kind), KeyModifiers.None);
+
+	[AvaloniaTest]
+	public async Task Middle_Press_On_A_Fold_Marker_Does_Not_Toggle_The_Fold()
+	{
+		var (window, view) = await SetupAsync();
+		var foldingMargin = view.Editor.TextArea.LeftMargins.OfType<FoldingMargin>().Single();
+		Control? Marker() => foldingMargin.GetVisualChildren().OfType<Control>().FirstOrDefault(c => c.IsVisible);
+		Marker().Should().NotBeNull("the folding margin must show a marker for the decompiled type");
+		var foldedBefore = view.FoldedFoldingCount;
+
+		await window.ClickAsync(Marker, MouseButton.Middle);
+		await Waiters.WaitForIdleAsync();
+		view.FoldedFoldingCount.Should().Be(foldedBefore, "a middle press on a fold marker must not toggle the fold");
+
+		// The same marker toggles on a left click, so the middle press above really reached a marker.
+		await window.ClickAsync(Marker, MouseButton.Left);
+		await Waiters.WaitForAsync(() => view.FoldedFoldingCount != foldedBefore,
+			description: "a left click on the fold marker to toggle the fold");
+	}
+}

ILSpy/TextView/DecompilerTextView.axaml.cs

@@ -259,6 +259,24 @@ namespace ICSharpCode.ILSpy.TextView
 				OnTextAreaPointerReleasedForReferenceClick,
 				RoutingStrategies.Bubble,
 				handledEventsToo: true);
+			// The gutter margins react to the left button only, like their WPF counterparts:
+			// AvaloniaEdit's line-number margin moves the caret and selects the line, and a fold
+			// marker toggles, on any button. Marking the press handled before it reaches the margin
+			// skips the margin's own press handler. The text area keeps seeing middle presses (a
+			// middle click on a reference opens it in a new tab).
+			Editor.TextArea.AddHandler(InputElement.PointerPressedEvent,
+				OnTextAreaPointerPressedForMargins,
+				RoutingStrategies.Tunnel);
+		}
+
+		void OnTextAreaPointerPressedForMargins(object? sender, PointerPressedEventArgs e)
+		{
+			if (e.GetCurrentPoint(Editor.TextArea).Properties.PointerUpdateKind == PointerUpdateKind.LeftButtonPressed)
+				return;
+			if (e.Source is Visual hit
+				&& hit.FindAncestorOfType<AbstractMargin>(includeSelf: true) is { } margin
+				&& Editor.TextArea.LeftMargins.Contains(margin))
+				e.Handled = true;
 		}
 
 		// Position of the last link-button press, in this control's coordinates; null while no