Batch the after-drop selection through SelectNodes

The drag-drop / file-drop callback selected the dropped assemblies with a
raw SelectedItems.Clear() + Add() loop, which flashes a transient empty
selection. The model has SelectNodes precisely to avoid that -- it brackets
the swap so the selection-changed fan-out fires once, with the final set,
never empty. A transient empty poisons the grid-sync deferred guard, after
which the tree stops following tab activation. Route the callback through
SelectNodes; a regression test asserts no empty SelectedItem is observed
mid-drop.

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

ILSpy.Tests/AssemblyList/DropSelectionBatchingTests.cs

@@ -0,0 +1,87 @@
+// 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.Collections.Generic;
+using System.ComponentModel;
+using System.Linq;
+using System.Threading.Tasks;
+
+using AwesomeAssertions;
+
+using ICSharpCode.ILSpyX.TreeView;
+
+using ILSpy.AssemblyTree;
+using ILSpy.TreeNodes;
+using ILSpy.Views;
+
+using Avalonia.Headless.NUnit;
+
+using NUnit.Framework;
+
+namespace ICSharpCode.ILSpy.Tests;
+
+/// <summary>
+/// Selecting the assemblies that were just dropped / reordered must go through the model's batched
+/// SelectNodes, not a raw Clear()+Add(). A raw swap flashes a transient EMPTY selection, which the
+/// model documents as poisoning the grid-sync deferred guard (the tree stops following tab
+/// activation). This pins that the drop-selection callback never emits an empty selection mid-change.
+/// </summary>
+[TestFixture]
+public class DropSelectionBatchingTests
+{
+	[AvaloniaTest]
+	public async Task Drop_Selection_Does_Not_Flash_An_Empty_Selection()
+	{
+		var (window, vm) = await TestHarness.BootAsync(3);
+		await window.WaitForComponent<AssemblyListPane>();
+		var model = vm.AssemblyTreeModel;
+		var listRoot = (AssemblyListTreeNode)model.Root!;
+
+		listRoot.SelectAssembliesAfterDrop.Should().NotBeNull(
+			"the pane wires the after-drop selection callback when it binds");
+
+		var assemblies = model.AssemblyList!.GetAssemblies();
+		assemblies.Length.Should().BeGreaterThanOrEqualTo(2);
+
+		// Start from a non-empty selection so a raw Clear() would transit through empty.
+		model.SelectedItem = listRoot.FindAssemblyNode(assemblies[0]);
+
+		var observed = new List<SharpTreeNode?>();
+		void OnChanged(object? _, PropertyChangedEventArgs e)
+		{
+			if (e.PropertyName == nameof(AssemblyTreeModel.SelectedItem))
+				observed.Add(model.SelectedItem);
+		}
+		model.PropertyChanged += OnChanged;
+		try
+		{
+			// Drive the after-drop selection the way AssemblyListTreeNode.Drop does.
+			listRoot.SelectAssembliesAfterDrop!(new[] { assemblies[0], assemblies[1] });
+		}
+		finally
+		{
+			model.PropertyChanged -= OnChanged;
+		}
+
+		observed.Should().NotContainNulls(
+			"the drop selection must not flash an empty SelectedItem mid-change");
+		observed.Should().ContainSingle(
+			"the batched selection change fans out exactly once, with the final set");
+		model.SelectedItems.Should().HaveCount(2, "both dropped assemblies end up selected");
+	}
+}

ILSpy/AssemblyTree/AssemblyListPane.axaml.cs

@@ -334,9 +334,10 @@ namespace ILSpy.AssemblyTree
 					.ToList();
 				if (nodes.Count == 0)
 					return;
-				model.SelectedItems.Clear();
-				foreach (var node in nodes)
-					model.SelectedItems.Add(node);
+				// Go through the batched setter, not a raw Clear()+Add(): the latter flashes a
+				// transient empty selection that poisons the grid-sync deferred guard (the tree
+				// stops following tab activation).
+				model.SelectNodes(nodes);
 			};
 		}