Don't recentre tree row on user click

Clicking an already-visible row was running ScrollIntoView+CenterRowInView via
the SelectionChanged → model PropertyChanged round-trip, yanking the viewport.
Add a clickInProgress flag separate from syncingSelection: clickInProgress is
set in OnTreeGridSelectionChanged and cleared at Background priority alongside
syncingSelection, so the model's two PropertyChanged notifications (Clear+Add
on SelectedItems) both bail out — but hyperlink / Back/Forward / search-hit
navigation, which never enters OnTreeGridSelectionChanged, still scrolls the
target into view.

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

ILSpy.Tests/AssemblyList/AssemblyTreeTests.cs

@@ -22,7 +22,12 @@ using System.Linq;
 using System.Threading.Tasks;
 using System.Xml.Linq;
 
+using Avalonia;
+using Avalonia.Controls;
+using Avalonia.Controls.DataGridHierarchical;
 using Avalonia.Headless.NUnit;
+using Avalonia.Threading;
+using Avalonia.VisualTree;
 
 using AwesomeAssertions;
 
@@ -30,9 +35,7 @@ using ICSharpCode.ILSpy.Properties;
 
 using ICSharpCode.Decompiler.TypeSystem;
 using ICSharpCode.ILSpyX;
-
-using Avalonia.Controls;
-using Avalonia.VisualTree;
+using ICSharpCode.ILSpyX.TreeView;
 
 using ILSpy;
 using ILSpy.AppEnv;
@@ -589,4 +592,75 @@ public class AssemblyTreeTests
 		node.LoadedAssembly.Should().BeSameAs(loaded);
 		node.IsSelected.Should().BeTrue();
 	}
+
+	[AvaloniaTest]
+	public async Task User_Click_On_Visible_Row_Does_Not_Recentre_Viewport()
+	{
+		var window = AppComposition.Current.GetExport<MainWindow>();
+		window.Show();
+		var vm = (MainWindowViewModel)window.DataContext!;
+		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 3);
+
+		var enumerable = vm.AssemblyTreeModel.FindNode<TypeTreeNode>(
+			"System.Linq", "System.Linq", "System.Linq.Enumerable");
+		enumerable.EnsureLazyChildren();
+		var ns = (NamespaceTreeNode)enumerable.Parent!;
+		ns.EnsureLazyChildren();
+		foreach (var child in ns.Children.OfType<TypeTreeNode>())
+			child.EnsureLazyChildren();
+
+		var pane = await window.WaitForComponent<AssemblyListPane>();
+		var grid = await pane.WaitForComponent<DataGrid>();
+
+		// Park the viewport mid-list so the next-clicked row sits at a non-zero offset. The
+		// regression would re-centre on click and snap the offset back; we want offset to
+		// stay put.
+		vm.AssemblyTreeModel.SelectedItem = enumerable;
+		await Waiters.WaitForAsync(() => ReferenceEquals(vm.AssemblyTreeModel.SelectedItem, enumerable));
+		for (int i = 0; i < 8; i++)
+		{
+			Dispatcher.UIThread.RunJobs();
+			await Task.Delay(25);
+		}
+		grid.UpdateLayout();
+
+		var scrollViewer = grid.GetVisualDescendants().OfType<ScrollViewer>().Single();
+		(scrollViewer.Extent.Height - scrollViewer.Viewport.Height).Should().BeGreaterThan(50,
+			"the grid must have something to scroll for this test to be meaningful");
+		scrollViewer.Offset = new Vector(scrollViewer.Offset.X, 50);
+		grid.UpdateLayout();
+		Dispatcher.UIThread.RunJobs();
+		scrollViewer.Offset.Y.Should().BeGreaterThan(5,
+			"the test scenario requires the viewport be parked mid-list");
+
+		// Pick a different visible row to click on; clicking an already-visible row must NOT
+		// move the viewport. Use the *bottom-most* visible row for a strict check — if the bug
+		// regressed, CenterRowInView would scroll it up to the middle and offset would change.
+		var candidateRow = grid.GetVisualDescendants().OfType<DataGridRow>()
+			.Where(r => !r.IsSelected
+				&& r.TranslatePoint(new Point(0, 0), scrollViewer) is { } p
+				&& p.Y >= 0 && p.Y + r.Bounds.Height <= scrollViewer.Viewport.Height)
+			.OrderByDescending(r => r.TranslatePoint(new Point(0, 0), scrollViewer)!.Value.Y)
+			.FirstOrDefault();
+		candidateRow.Should().NotBeNull("the test needs a visible non-selected row to click");
+
+		var offsetBefore = scrollViewer.Offset.Y;
+
+		// Real pointer click — setting SelectedItem programmatically would fire DataGrid's
+		// internal ScrollIntoView too, which a real user click does not.
+		var rowCentre = candidateRow!.TranslatePoint(
+			new Point(candidateRow.Bounds.Width / 2, candidateRow.Bounds.Height / 2),
+			window)!.Value;
+		global::Avalonia.Headless.HeadlessWindowExtensions.MouseDown(window, rowCentre, global::Avalonia.Input.MouseButton.Left);
+		global::Avalonia.Headless.HeadlessWindowExtensions.MouseUp(window, rowCentre, global::Avalonia.Input.MouseButton.Left);
+
+		for (int i = 0; i < 8; i++)
+		{
+			Dispatcher.UIThread.RunJobs();
+			await Task.Delay(25);
+		}
+
+		scrollViewer.Offset.Y.Should().BeApproximately(offsetBefore, 1.0,
+			"clicking an already-visible row must not move the viewport");
+	}
 }

ILSpy/AssemblyTree/AssemblyListPane.axaml.cs

@@ -132,9 +132,26 @@ namespace ILSpy.AssemblyTree
 		}
 
 		// DataGrid.ScrollIntoView only brings the row to the nearest viewport edge; we want
-		// the row centred so the user's eye lands on it.
+		// the row centred so the user's eye lands on it. Skip the move when the row is already
+		// fully in view — re-centring an in-view row would yank the viewport on every selection
+		// (e.g. user clicks a visible row, or Ctrl+O selects a freshly-loaded top-level entry).
 		void CenterRowInView(HierarchicalNode node)
 		{
+			var scrollViewer = TreeGrid.GetVisualDescendants().OfType<ScrollViewer>().FirstOrDefault();
+			if (scrollViewer is null)
+				return;
+
+			// Cheap pre-check: if the row's already realised AND fully visible, leave the
+			// viewport alone. ScrollIntoView (next call) would otherwise drag it to the edge,
+			// and our centring step would then drag it again.
+			var existingRow = TreeGrid.GetVisualDescendants().OfType<DataGridRow>()
+				.FirstOrDefault(r => ReferenceEquals(r.DataContext, node));
+			if (existingRow is { IsVisible: true }
+				&& existingRow.TranslatePoint(new Point(0, 0), scrollViewer) is { } existingTop
+				&& existingTop.Y >= 0
+				&& existingTop.Y + existingRow.Bounds.Height <= scrollViewer.Viewport.Height)
+				return;
+
 			TreeGrid.ScrollIntoView(node, TreeGrid.Columns[0]);
 			// ScrollIntoView only changes ScrollViewer.Offset; the row becomes a realised
 			// DataGridRow during the next layout pass. Force it now so we can read the row's
@@ -146,10 +163,6 @@ namespace ILSpy.AssemblyTree
 			if (row is null)
 				return;
 
-			var scrollViewer = TreeGrid.GetVisualDescendants().OfType<ScrollViewer>().FirstOrDefault();
-			if (scrollViewer is null)
-				return;
-
 			var rowTopInViewer = row.TranslatePoint(new Point(0, 0), scrollViewer);
 			if (rowTopInViewer is null)
 				return;