Reactor TreeView does not render per-node ContentElement

Minimal repro for Microsoft.UI.Reactor (experimental).

Summary

A TreeView whose TreeViewNodeData nodes carry a ContentElement shows blank rows — the per-node content (a Button, TextBlock, CheckBox, …) is never displayed. ListView / GridView with element items render fine; only TreeView is affected.

Repro

dotnet build .\ReactorTreeViewContentBug.csproj -c Debug -p:Platform=x64
$exe = ".\bin\x64\Debug\net10.0-windows10.0.22621.0\win-x64\ReactorTreeViewContentBug.exe"

& $exe            # window: a BLANK TreeView above a working ListView (same content)
& $exe -- --check # headless: detects each control's content, prints results, exit 0/2

Program.cs puts the same kind of element content into a TreeView and a ListView side by side:

// TreeView — per-node ContentElement (BLANK at runtime)
TreeView(new[]
{
    new TreeViewNodeData("n1") { ContentElement = Button("TreeView: I am a Button"), IsExpanded = true },
    new TreeViewNodeData("n2") { ContentElement = TextBlock("TreeView: I am a TextBlock") },
    new TreeViewNodeData("n3") { ContentElement = CheckBox(true, label: "TreeView: I am a CheckBox") },
})

// ListView — element items (renders correctly; the contrast)
ListView(
    Button("ListView: I am a Button"),
    TextBlock("ListView: I am a TextBlock"),
    CheckBox(true, label: "ListView: I am a CheckBox"))

Expected vs. actual

Control	Same element content	Expected	Actual
TreeView	Button / TextBlock / CheckBox	visible	all rows blank
ListView	Button / TextBlock / CheckBox	visible	visible ✅

--check output (exit code 2):

[check] ListView element-item content rendered:  True  (expected True)
[check] TreeView ContentElement content rendered: False  (expected True)
[check] BUG: TreeView ContentElement not rendered — a ListView with the same element content does render.

The check detects each item by the unique label text it carries (robust because the TreeView's internal TreeViewList derives from ListView). The TreeView content elements are mounted as TreeViewNode.Content but nothing hosts them, so they never enter the visual tree; the ListView content does.

Root cause

In Reconciler.Mount.cs MountTreeView (and the experimental V1Protocol/.../ChildrenStrategy.cs TreeChildren), when any node has a ContentElement:

treeView.ItemTemplate = SharedContentControlTemplate.Value; // "<ContentControl .../>", no Content binding
...
node.Content = Mount(data.ContentElement, requestRerender);  // a UIElement

SharedContentControlTemplate (Reconciler.cs) is an empty ContentControl shell:

<ContentControl HorizontalContentAlignment='Stretch' VerticalContentAlignment='Stretch'/>

For ListView / GridView, the framework wires ContainerContentChanging (HandleTemplatedContainerContentChanging) to imperatively push node.Content into that ContentControl. The TreeView path (MountTreeView / UpdateTreeView / TreeChildren) never does, so the shell's Content stays null and nothing is shown.

The text path works because the non-ContentElement template does bind: <TextBlock Text='{Binding Content.Content}'/>.

Suggested fix

Either give the TreeView shell template a Content binding so it resolves the node's content (the item template's DataContext is the TreeViewNode):

<ContentControl Content='{Binding Content}' HorizontalContentAlignment='Stretch' VerticalContentAlignment='Stretch'/>

…or populate the TreeView's node containers imperatively the way the ListView/GridView path already does.

Workaround (consumer side)

Override ItemTemplate after Reactor sets it — UpdateTreeView never touches ItemTemplate, so it sticks:

TreeView(nodes).Set(tv => tv.ItemTemplate = (DataTemplate)XamlReader.Load(
    "<DataTemplate xmlns='http://schemas.microsoft.com/winfx/2006/xaml/presentation'>" +
    "<ContentControl Content='{Binding Content}'/></DataTemplate>"));

Environment

• microsoft-ui-reactor: experimental (April 2026), built from source via ProjectReference
• Microsoft.WindowsAppSDK 2.0.1, net10.0-windows10.0.22621.0, x64, unpackaged self-contained
• .NET 10 SDK, Windows 11