Reuse the metadata grid tab across node clicks

Picking DOS Header → COFF Header → DOS Header was opening three
dockables because ShowCustomTab unconditionally added a fresh tab. The
docking host now searches for an open tab of the same concrete type,
copies the new node's tab state onto it, and re-activates it — same
shape as how the single decompiler tab gets reused across tree-node
selections.

Assisted-by: Claude:claude-opus-4-7:Claude Code

ILSpy.Tests/Metadata/PEHeaderTreeTests.cs

@@ -105,6 +105,46 @@ public class PEHeaderTreeTests
 		lastRow.Member.Should().Be("e_lfanew");
 	}
 
+	[AvaloniaTest]
+	public async Task Selecting_A_Second_Metadata_Node_Reuses_The_Existing_Grid_Tab()
+	{
+		// Clicking DOS Header → COFF Header → DOS Header should leave the dock with a
+		// single metadata tab whose state has been updated each time, mirroring how the
+		// decompiler tab reuses itself across tree-node selections. Otherwise every click
+		// piles up a new dockable and the tab strip grows without bound.
+
+		var window = AppComposition.Current.GetExport<MainWindow>();
+		window.Show();
+		var vm = (MainWindowViewModel)window.DataContext!;
+		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 1);
+
+		var coreLibName = typeof(object).Assembly.GetName().Name!;
+		var assemblyNode = vm.AssemblyTreeModel.FindNode<AssemblyTreeNode>(coreLibName);
+		assemblyNode.EnsureLazyChildren();
+		var metadataNode = assemblyNode.Children.OfType<MetadataTreeNode>().Single();
+		metadataNode.EnsureLazyChildren();
+
+		var dosNode = metadataNode.Children.OfType<DosHeaderTreeNode>().Single();
+		var coffNode = metadataNode.Children.OfType<CoffHeaderTreeNode>().Single();
+
+		vm.AssemblyTreeModel.SelectNode(dosNode);
+		var firstTab = await vm.DockWorkspace.WaitForMetadataTabAsync();
+		firstTab.Title.Should().Be("DOS Header");
+
+		vm.AssemblyTreeModel.SelectNode(coffNode);
+		var secondTab = await vm.DockWorkspace.WaitForMetadataTabAsync();
+		secondTab.Should().BeSameAs(firstTab, "metadata clicks reuse the existing grid tab");
+		secondTab.Title.Should().Be("COFF Header");
+
+		vm.AssemblyTreeModel.SelectNode(dosNode);
+		var thirdTab = await vm.DockWorkspace.WaitForMetadataTabAsync();
+		thirdTab.Should().BeSameAs(firstTab);
+		thirdTab.Title.Should().Be("DOS Header");
+
+		var documents = ((global::ILSpy.Docking.ILSpyDockFactory)vm.DockWorkspace.Factory).Documents!;
+		documents.VisibleDockables!.OfType<global::ILSpy.ViewModels.MetadataTablePageModel>().Should().ContainSingle();
+	}
+
 	[AvaloniaTest]
 	public async Task CoffHeaderTreeNode_Opens_A_DataGrid_Tab_With_Machine_And_Characteristics_Rows()
 	{

ILSpy/Docking/DockWorkspace.cs

@@ -282,11 +282,49 @@ namespace ILSpy.Docking
 		{
 			if (factory.Documents == null)
 				return;
+
+			// Reuse an existing tab of the same concrete type — same model the decompiler tab
+			// uses, so DOS Header → COFF Header → DOS Header doesn't pile up three dockables.
+			// Prefer the active dockable when it's already the right type so explicit
+			// "switch tabs and pick a node" interactions land where the user expects.
+			if (FindReusableTab(tab.GetType()) is { } existing)
+			{
+				CopyTabState(tab, existing);
+				factory.SetActiveDockable(existing);
+				factory.SetFocusedDockable(factory.Documents, existing);
+				return;
+			}
+
 			factory.AddDockable(factory.Documents, tab);
 			factory.SetActiveDockable(tab);
 			factory.SetFocusedDockable(factory.Documents, tab);
 		}
 
+		TabPageModel? FindReusableTab(Type tabType)
+		{
+			if (factory.Documents == null)
+				return null;
+			if (factory.Documents.ActiveDockable is TabPageModel active && active.GetType() == tabType)
+				return active;
+			if (factory.Documents.VisibleDockables == null)
+				return null;
+			foreach (var d in factory.Documents.VisibleDockables)
+				if (d is TabPageModel m && m.GetType() == tabType)
+					return m;
+			return null;
+		}
+
+		static void CopyTabState(TabPageModel source, TabPageModel target)
+		{
+			if (source is MetadataTablePageModel newMeta && target is MetadataTablePageModel oldMeta)
+			{
+				oldMeta.Title = newMeta.Title;
+				oldMeta.Columns = newMeta.Columns;
+				oldMeta.Items = newMeta.Items;
+				oldMeta.ScrollToRow = newMeta.ScrollToRow;
+			}
+		}
+
 		public DecompilerTabPageModel? ActiveDecompilerTab => GetDecompilerContentTab();
 
 		/// <summary>