Avalonia: Selection and ObservableCollection issue

[Onebeld]

Reproduction steps (i used ControlCatalog):

1. Add a "Delete" button to the TabControl page:

<Button Command="{Binding DeleteTab}" Content="Delete"/>

1. Change List to ObservableCollection:
   ```c#
   private class PageViewModel : ReactiveObject
   {
   private ObservableCollection _tabs;

   private Dock _tabPlacement;

   public ObservableCollection Tabs
   {
   get => _tabs;
   set => this.RaiseAndSetIfChanged(ref _tabs, value);
   }

   public Dock TabPlacement
   {
   get { return _tabPlacement; }
   set { this.RaiseAndSetIfChanged(ref _tabPlacement, value); }
   }

   private void DeleteTab()
   {
   Tabs.RemoveAt(0);
   }
   }
   ```

2. Launch the application, click on the button and this will happen:
   

I'm guessing this was broken after the global changes related to the SelectionModel, since TabControl hasn't changed

Used the most recent preview of Avalonia

[grokys]

Thanks for the excellent repro! On it now.

[Onebeld]

Perhaps this information can be useful.

If you delete any item before the selection, shows the previous item, not the selected one before deletion.
The deleted item is displayed here:

And here is the previous one:

When removing the last item, it highlights the first one (I don't think this will be critical).