Lay document tabs out on multiple rows

#3753: when many documents are open, the tab strip scrolls a single row,
hiding most tabs behind the overflow scroller.

Dock's DocumentTabStrip hardcodes a horizontal StackPanel inside a
PART_ScrollViewer in its theme template and ignores an ItemsPanel set via
a Style, so a MultiRowTabStripBehavior sets the WrapPanel ItemsPanel
directly and disables the scroll-viewer's horizontal scrolling, giving
the WrapPanel a bounded width so tabs wrap onto new rows. Enabled on the
document strip (not tool tabs) from App.axaml.

Drag-reorder across rows is not yet validated (Dock's drop-position logic
assumes a single horizontal strip), so this stays an opt-in prototype for
now rather than a closing fix for the issue.

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

ILSpy.Tests/Docking/MultiRowTabStripTests.cs

@@ -0,0 +1,78 @@
+// 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.
+
+using System.Linq;
+using System.Threading.Tasks;
+
+using Avalonia.Controls;
+using Avalonia.Headless.NUnit;
+using Avalonia.Threading;
+using Avalonia.VisualTree;
+
+using AwesomeAssertions;
+
+using Dock.Avalonia.Controls;
+
+using ILSpy.AppEnv;
+using ILSpy.TextView;
+using ILSpy.ViewModels;
+using ILSpy.Views;
+
+using NUnit.Framework;
+
+namespace ICSharpCode.ILSpy.Tests.Docking;
+
+/// <summary>
+/// PROTOTYPE check: with MultiRowTabStripBehavior enabled (App.axaml), many document tabs flow onto
+/// multiple rows (a WrapPanel) instead of a single scrolling row.
+/// </summary>
+[TestFixture]
+public class MultiRowTabStripTests
+{
+	[AvaloniaTest]
+	public async Task Many_Document_Tabs_Wrap_To_Multiple_Rows()
+	{
+		var window = AppComposition.Current.GetExport<MainWindow>();
+		window.Show();
+		var vm = (MainWindowViewModel)window.DataContext!;
+		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 1);
+
+		for (int i = 0; i < 40; i++)
+			vm.DockWorkspace.OpenNewTab(new DecompilerTabPageModel { Title = $"Tab number {i:00}" });
+
+		for (int i = 0; i < 12; i++)
+		{
+			Dispatcher.UIThread.RunJobs();
+			window.UpdateLayout();
+			await Task.Delay(20);
+		}
+
+		var strip = window.GetVisualDescendants().OfType<DocumentTabStrip>().FirstOrDefault();
+		strip.Should().NotBeNull("the document tab strip must be realised");
+
+		strip!.GetVisualDescendants().OfType<WrapPanel>().Should().NotBeEmpty(
+			"the behaviour must swap the strip's ItemsPanel to a WrapPanel");
+
+		var rows = strip.GetVisualDescendants().OfType<DocumentTabStripItem>()
+			.Where(it => it.Bounds.Width > 0)
+			.Select(it => System.Math.Round(it.Bounds.Y))
+			.Distinct().ToList();
+
+		rows.Count.Should().BeGreaterThan(1, "40 tabs must wrap onto more than one row");
+	}
+}

ILSpy/App.axaml

@@ -285,6 +285,10 @@
         <Style Selector="dockControls|DocumentTabStrip, dockControls|ToolTabStrip">
             <Setter Property="Background" Value="{DynamicResource ILSpy.DockTabStripBackground}" />
         </Style>
+        <!-- PROTOTYPE: lay document tabs out on multiple rows instead of a single scrolling row. -->
+        <Style Selector="dockControls|DocumentTabStrip">
+            <Setter Property="themes:MultiRowTabStripBehavior.Enable" Value="True" />
+        </Style>
         <!-- The full-width accent line under the document tab strip is the DocumentControl
              template's Panel#PART_DocumentSeperator. The Simple theme fills it with the blue
              active-indicator brush whenever the dock is :active; re-assert it at app level (app

ILSpy/Themes/MultiRowTabStripBehavior.cs

@@ -0,0 +1,84 @@
+// 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.
+
+using System.Linq;
+
+using Avalonia;
+using Avalonia.Controls;
+using Avalonia.Controls.Primitives;
+using Avalonia.Controls.Templates;
+using Avalonia.Layout;
+using Avalonia.VisualTree;
+
+using Dock.Avalonia.Controls;
+
+namespace ILSpy.Themes
+{
+	/// <summary>
+	/// PROTOTYPE: makes the document tab strip lay its tabs out on multiple rows instead of a single
+	/// scrolling row. Dock's <see cref="DocumentTabStrip"/> hardcodes a horizontal StackPanel inside a
+	/// PART_ScrollViewer in its theme template and ignores an ItemsPanel set via a Style, so this
+	/// behaviour sets the WrapPanel ItemsPanel directly and disables the scroll-viewer's horizontal
+	/// scrolling (so the WrapPanel gets a bounded width and actually wraps). Toggle via the attached
+	/// Enable property in App.axaml.
+	/// </summary>
+	public static class MultiRowTabStripBehavior
+	{
+		public static readonly AttachedProperty<bool> EnableProperty =
+			AvaloniaProperty.RegisterAttached<DocumentTabStrip, bool>(
+				"Enable",
+				typeof(MultiRowTabStripBehavior));
+
+		public static void SetEnable(DocumentTabStrip element, bool value)
+			=> element.SetValue(EnableProperty, value);
+
+		public static bool GetEnable(DocumentTabStrip element)
+			=> element.GetValue(EnableProperty);
+
+		static MultiRowTabStripBehavior()
+		{
+			EnableProperty.Changed.AddClassHandler<DocumentTabStrip>(OnEnableChanged);
+		}
+
+		static void OnEnableChanged(DocumentTabStrip strip, AvaloniaPropertyChangedEventArgs e)
+		{
+			if (e.NewValue is not true)
+				return;
+
+			// The presenter builds its panel from ItemsPanel; setting it here (before/at styling time)
+			// makes the tabs flow through a WrapPanel.
+			strip.ItemsPanel = new FuncTemplate<Panel?>(
+				() => new WrapPanel { Orientation = Orientation.Horizontal });
+
+			// The PART_ScrollViewer only exists once the template is applied. Disabling its horizontal
+			// scroll constrains the WrapPanel to the strip width so it wraps onto new rows.
+			strip.TemplateApplied += (_, _) => ConstrainScroller(strip);
+			ConstrainScroller(strip);
+		}
+
+		static void ConstrainScroller(DocumentTabStrip strip)
+		{
+			var scroller = strip.GetVisualDescendants().OfType<ScrollViewer>()
+				.FirstOrDefault(s => s.Name == "PART_ScrollViewer");
+			if (scroller is null)
+				return;
+			scroller.HorizontalScrollBarVisibility = ScrollBarVisibility.Disabled;
+			scroller.VerticalScrollBarVisibility = ScrollBarVisibility.Auto;
+		}
+	}
+}