TreeViewItem.IsExpanded two-way x:Bind binding does not work in TreeView.ItemTemplate

[PetrAltman]

Describe the bug

TreeViewItem.IsExpanded with {x:Bind ..., Mode=TwoWay} does not work when used inside a TreeView.ItemTemplate. The binding from ViewModel → View is ignored — programmatically setting the bound property does not always expand/collapse the tree node.

The issue is likely related to virtualization — when items are recycled, the IsExpanded state from the ViewModel is not applied to the newly materialized TreeViewItem.

Why is this important?

I'm building a client application that displays file diffs in a hierarchical tree structure (folders → files → diff content). Users need to expand/collapse tree nodes both manually and programmatically (e.g. "Expand All" / "Collapse All" buttons).

Steps to reproduce the bug

1. Create a TreeView with a hierarchical ItemTemplate
2. Inside the DataTemplate, place a TreeViewItem with IsExpanded="{x:Bind IsExpanded, Mode=TwoWay}"
3. The ViewModel property implements INotifyPropertyChanged
4. Set IsExpanded = true on a ViewModel item programmatically
5. Observe that the tree nodes do not expand

<TreeView ItemsSource="{x:Bind Items,Mode=OneWay}">
    <TreeView.ItemTemplate>
        <DataTemplate x:DataType="vm:TreeItemViewModel">
            <TreeViewItem ItemsSource="{x:Bind Items}" 
                          IsExpanded="{x:Bind IsExpanded, Mode=TwoWay}">
                <TextBlock Text="{x:Bind Name}"/>
            </TreeViewItem>
        </DataTemplate>
    </TreeView.ItemTemplate>
</TreeView>

Actual behavior

The IsExpanded binding from ViewModel → View has sometimes no effect. The tree nodes do not expand/collapse reliable when the bound property changes.

Expected behavior

Setting IsExpanded = true on the ViewModel should expand the corresponding TreeViewItem, and collapsing a node in the UI should set the ViewModel property to false.

Screenshots

No response

NuGet package version

WinUI 3 - Windows App SDK 1.8.3: 1.8.251106002

Windows version

Windows 11 (24H2): Build 26100

Additional context

No response

[ayushjai19]

This relates to #10309