[Bug]: MewDock: TreeView (and other composite content) renders empty inside a tool pane; "visual is not connected to a window" on mouse move

[rkttu]

Summary

A TreeView placed inside a MewDock tool pane renders no rows (the pane is
blank). The same TreeView renders correctly as a plain Window's content.
A TextBlock renders fine both as the dock's CenterContent and as tool-pane
content, so only composite/virtualized controls appear to be affected.

In a variant where AddToolPane is called after the window is loaded,
moving the mouse over the (empty) tree pane throws:

System.InvalidOperationException: The visual is not connected to a window.
   at Aprillz.MewUI.MouseEventArgs.GetPosition(UIElement relativeTo)
   at Aprillz.MewUI.Controls.TreeView.OnMouseMove(MouseEventArgs e)
   at Aprillz.MewUI.Platform.Win32.Win32WindowBackend.HandleMouseMove(IntPtr lParam)
   at Aprillz.MewUI.Platform.Win32.Win32WindowBackend.ProcessMessage(UInt32 msg, IntPtr wParam, IntPtr lParam)
   at Aprillz.MewUI.Platform.Win32.Win32PlatformHost.WndProc(IntPtr hWnd, UInt32 msg, IntPtr wParam, IntPtr lParam)

Environment

• Aprillz.MewUI 0.18.1, Aprillz.MewUI.MewDock 0.18.1 (all-in-one metapackage)
• .NET SDK 10.0.301, target net10.0
• Windows 11, Win32 platform host + Direct2D backend

Minimal reproduction

dotnet run repro.cs

#:sdk Microsoft.NET.Sdk
#:property OutputType=WinExe
#:property TargetFramework=net10.0
#:package Aprillz.MewUI@0.18.1
#:package Aprillz.MewUI.MewDock@0.18.1

using Aprillz.MewUI;
using Aprillz.MewUI.Controls;
using Aprillz.MewUI.MewDock;

if (OperatingSystem.IsWindows()) { Win32Platform.Register(); Direct2DBackend.Register(); }
ThemeManager.Default = ThemeVariant.Dark;

var root = new Node("Project (this row is expected to be visible)", new Node("Child.cs"));
var tree = new TreeView().ItemHeight(22).Indent(16);
tree.ItemsSource = TreeItemsView.Create(
    new[] { root }, n => n.Kids, n => n.Text, n => n.Text, n => n.Kids.Count > 0);

var manager = new DockingManager();
manager.CenterContent = new TextBlock { Text = "Centre (document area)", FontSize = 16 };
manager.AddToolPane("Tree Pane (BUG: empty)", tree, DockEdge.Left, "tree");
manager.AddToolPane("Text Pane (OK)",
    new TextBlock { Text = "A TextBlock in a tool pane renders.", FontSize = 13 },
    DockEdge.Bottom, "text");

Application.Run(new Window()
    .Title("MewDock repro: TreeView tool pane is empty")
    .Resizable(760, 500)
    .Content(manager));

sealed class Node
{
    public string Text { get; }
    public List<Node> Kids { get; } = new();
    public Node(string t, params Node[] k) { Text = t; Kids.AddRange(k); }
}

Expected

The left tool pane shows the tree ("Project …", expandable to "Child.cs"),
like the same TreeView does as a plain window's content.

Actual

The left tool pane is empty; the bottom TextBlock tool pane and the
centre TextBlock render normally.

Notes

• Reproduced with both the direct AddToolPane(...) API and the
  WithContentFactory(...) + LoadLayout(json) path (as in samples/MewDock.Sample).
• The identical TreeView renders fine in new Window().Content(new Border().Child(tree)).
• A StackPanel of Button rows in a tool pane is also blank, so this looks
  like it affects container/virtualized content in tool panes generally, not
  TreeView specifically.

[al6uiz]

It has been released in v0.19.1. Please check it.