Keep the hover popup open while the user interacts with it

Selecting text inside the popup closed it: the selection drag captures
the pointer, so sweeping past the popup edge fired the content's
PointerExited with IsPointerOver already false. The close veto now
covers pointer-over, keyboard-focus-within (so a finished selection
survives for Ctrl+C, as the WPF tooltip did), and pointer capture held
inside the popup; only a document change force-closes.

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

ILSpy.Tests/Editor/DocumentationLinkTests.cs

@@ -104,6 +104,58 @@ public class DocumentationLinkTests
 			"the hosting popup closes itself when a link is followed");
 	}
 
+	[AvaloniaTest]
+	public async Task Pressing_Inside_The_Popup_To_Select_Does_Not_Close_It()
+	{
+		var (window, vm) = await TestHarness.BootAsync();
+		var coreLibName = typeof(object).Assembly.GetName().Name!;
+		var stringNode = vm.AssemblyTreeModel.FindNode<TypeTreeNode>(coreLibName, "System", "System.String");
+		vm.AssemblyTreeModel.SelectNode(stringNode);
+		await vm.DockWorkspace.WaitForDecompiledTextAsync();
+		window.UpdateLayout();
+
+		var view = window.GetVisualDescendants().OfType<DecompilerTextView>().First();
+		var renderer = new DocumentationRenderer(
+			new CSharpAmbience(),
+			new FontFamily("Consolas, Menlo, Monospace"),
+			12);
+		renderer.AddXmlDocumentation(
+			"""<summary>Some text the user may want to select and copy.</summary>""",
+			declaringEntity: null,
+			resolver: _ => null);
+		var content = renderer.CreateView();
+		view.OpenRichPopup(content);
+		AvaloniaHeadlessPlatform.ForceRenderTimerTick();
+		Avalonia.Threading.Dispatcher.UIThread.RunJobs();
+		window.UpdateLayout();
+
+		var popup = view.FindControl<Avalonia.Controls.Primitives.Popup>("RichHoverPopup")!;
+		popup.IsOpen.Should().BeTrue("setup precondition — the popup must be open before the press");
+
+		var paragraph = content.GetVisualDescendants().OfType<SelectableTextBlock>().First();
+		var inside = paragraph.TranslatePoint(
+			new Point(10, paragraph.Bounds.Height / 2), window)!.Value;
+
+		window.MouseDown(inside, MouseButton.Left);
+		popup.IsOpen.Should().BeTrue("holding LMB inside the popup starts a text selection, not a dismiss");
+		window.MouseMove(inside + new Point(25, 0));
+		popup.IsOpen.Should().BeTrue("dragging a selection inside the popup must not close it");
+
+		// Selecting to the end of a line routinely sweeps the pointer past the popup edge
+		// while the button is still held — the popup must survive that.
+		var farOutside = inside + new Point(600, 80);
+		window.MouseMove(farOutside);
+		popup.IsOpen.Should().BeTrue("a selection drag sweeping past the popup edge must not close it");
+
+		// Finish the selection back over the popup and release: the pointer is still over the
+		// popup, so it stays open for the copy. (Releasing far outside the popup is left to
+		// normal leave-to-close behaviour, which differs by platform focus model.)
+		window.MouseMove(inside + new Point(40, 0));
+		window.MouseUp(inside + new Point(40, 0), MouseButton.Left);
+		Avalonia.Threading.Dispatcher.UIThread.RunJobs();
+		popup.IsOpen.Should().BeTrue("the selection just finished over the popup — it stays for the copy");
+	}
+
 	[AvaloniaTest]
 	public void Unresolvable_Cref_Falls_Back_To_Plain_Text()
 	{

ILSpy/TextView/DecompilerTextView.axaml.cs

@@ -850,10 +850,14 @@ namespace ILSpy.TextView
 		{
 			if (richPopup.IsOpen)
 			{
-				// TODO: refuse to close while the popup has keyboard focus (the user is
-				// reading / clicking links inside it). Not tracked yet — for now the popup
-				// is always replaceable.
-				_ = mouseClick;
+				// While the pointer is over the popup or keyboard focus is inside it (the
+				// user is reading, selecting text to copy, or reaching for a link), only a
+				// forced close (document change) may take it down.
+				if (!mouseClick && richPopup.Child is { } child
+					&& (child.IsPointerOver || child.IsKeyboardFocusWithin))
+				{
+					return false;
+				}
 				CloseRichPopup();
 				return true;
 			}
@@ -907,7 +911,7 @@ namespace ILSpy.TextView
 			ToolTip.SetIsOpen(Editor.TextArea.TextView, true);
 		}
 
-		void OpenRichPopup(Control content)
+		internal void OpenRichPopup(Control content)
 		{
 			richPopup.Child = content;
 			// While the pointer is over the popup, the editor no longer receives the moves
@@ -920,6 +924,14 @@ namespace ILSpy.TextView
 
 		void OnRichPopupContentPointerExited(object? sender, PointerEventArgs e)
 		{
+			// A selection drag that sweeps past the popup edge keeps the pointer captured
+			// inside the popup — that's a wide swipe, not a leave. The over/focus vetoes in
+			// TryCloseExistingPopup don't see the capture, so check it here.
+			if (e.Pointer.Captured is Visual captured && richPopup.Child is { } child
+				&& (ReferenceEquals(captured, child) || captured.GetVisualAncestors().Contains(child)))
+			{
+				return;
+			}
 			TryCloseExistingPopup(mouseClick: false);
 		}