TreeViewItem Collapses When Its Items Are Removed Until Empty in WinUI 3

[datskiy]

Describe the bug

Removing items sequentially from an expanded TreeViewItem until it is empty causes it to collapse. When items are added back, the parent TreeViewItem unexpectedly remains collapsed. This behavior occurs when the underlying ObservableCollection emits a Remove event for the last item.

A Workaround
If the last item is removed using the Clear method instead of RemoveAt, the ObservableCollection will emit the Reset event, which will not cause this issue. However, I still find it very unintuitive and it is particularly inconvenient when the source collection is controlled by a 3rd party library, like DynamicData.

FYI: if the parent TreeViewItem was collapsed before calling Clear, it will remain collapsed after as expected, so there seems to be no issue with that.

Steps to reproduce the bug

1. Run the project: https://github.com/datskiy/TreeViewCollapsingIssue
2. Observe that the parent TreeViewItem is collapsed and start clicking the 'Remove Item' button until all of them are removed.
3. Click the 'Restore Item' button to add one item back and observe that the parent TreeViewItem remains collapsed.

Expected behavior

The parent TreeViewItem should automatically expand when items are re-added to the source ObservableCollection if it was expanded before removing all the items, like it happens when the Clear method is used.

Screenshots

No response

NuGet package version

WinUI 3 - Windows App SDK 1.6.3: 1.6.241114003

Windows version

Windows 11 (24H2): Build 26100

Additional context

The IsExpanded property is set as True in XAML.

Source code: https://github.com/datskiy/TreeViewCollapsingIssue

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• TreeViewItem does not handle IsExpanded binding correctly (#10140), similarity score: 0.82

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[Mangepange]

I experience a similar issue when I reset the source collection in my viewmodel. I perform some sorting logic, and sometimes some of the nodes are collapsed afterwards. Using Remove/Insert on my ObsservableCollection instead of resetting the whole collection makes no difference.

Seems like there are several similiar registered issues out there.