Unable to bind to SelectionModel.SelectedItems or SelectedIndexes

[grokys]

Describe the bug

Reported by a customer:

Binding ItemsControl.ItemsSource to SelectionModel.SelectedItems or SelectionModel.SelectedIndexes does not work.

SelectionModel raises a PropertyChanged event on SelectedItems and SelectedIndexes, but because the instance of the collection hasn't changed, property system doesn't consider the value to have changed, so consequently the bound ItemsControl doesn't register the change. This behaviour comes from WinUI which was the original source of our SelectionModel (though they have diverged significantly since the initial port):

https://github.com/microsoft/microsoft-ui-xaml/blob/winui3/release/1.4-stable/controls/dev/Repeater/SelectionModel.cpp#L591-L592

I assume that the WinUI SelectionModel was written without the expectation that someone would bind to the selected collections on a SelectionModel.

To Reproduce

<Grid ColumnDefinitions="*,*">
    <ListBox Name="lb" ItemsSource="{Binding}" SelectionMode="Multiple"/>
    <ListBox Grid.Column="1" ItemsSource="{Binding #lb.Selection.SelectedIndexes}"/>
</Grid>

Expected behavior

SelectionModel.SelectedItems or SelectionModel.SelectedIndexes and should follow the usual semantics for property and collection changes, not the strange semantics we have inherited from WinUI.

Avalonia version

all

OS

No response

Additional context

No response