Selection Model Issues

[danwalmsley]

• Always selected mode - Add 3 items, select the last item... remove the middle item... this leaves no item selected.
• Crash when removing all items, and then inserting at position 0.
  

-- more to come.

[grokys]

Both of these issues are related but the second is the most interesting (and hardest to fix). The following unit test demonstrates the problem:

[Fact]
public void Binding_SelectedIndex_Does_Not_Cause_ArgumentOutOfRange()
{
    // Issue #4496 (part 2)
    var items = new ObservableCollection<string>();

    var other = new ListBox
    {
        Template = Template(),
        Items = items,
        SelectionMode = SelectionMode.AlwaysSelected,
    };

    var target = new ListBox
    {
        Template = Template(),
        Items = items,
        [!ListBox.SelectedIndexProperty] = other[!ListBox.SelectedIndexProperty],
    };

    target.ApplyTemplate();
    target.Presenter.ApplyTemplate();
    other.ApplyTemplate();
    other.Presenter.ApplyTemplate();

    items.Add("Foo");
}

What happens is:

• target has SelectedIndex bound to other.SelectedIndex
• An item is added: other auto-selects the item
• The binding takes effect, setting target.SelectedIndex = 0
• target's selection model recieves CollectionChanged, saying that an item was inserted at index 0
• So it adjusts the index of the selected item by adding 1 to it, to an index that doesn't exist