Calculator.exe is setting selected index to an invalid value

We're seeing from crash dumps that Calculator is setting selected index on a selector-derived type to a value that is outside the valid range.

The actual failure occurs in the XAML code, where the call stack looks like this:
0 Windows_UI_Xaml!DirectUI::Selector::OnSelectedIndexChanged
1 Windows_UI_Xaml!DirectUI::Selector::OnPropertyChanged2
2 Windows_UI_Xaml!DirectUI::ComboBox::OnPropertyChanged2
3 Windows_UI_Xaml!DirectUI::DependencyObject::NotifyPropertyChanged
4 Windows_UI_Xaml!CDependencyObject::NotifyPropertyChanged
5 Windows_UI_Xaml!CDependencyObject::SetEffectiveValue
6 Windows_UI_Xaml!CDependencyObject::UpdateEffectiveValue
7 Windows_UI_Xaml!CDependencyObject::SetValue
8 Windows_UI_Xaml!CUIElement::SetValue
9 Windows_UI_Xaml!CFrameworkElement::SetValue
10 Windows_UI_Xaml!CControl::SetValue
11 Windows_UI_Xaml!CItemsControl::SetValue
12 Windows_UI_Xaml!DirectUI::DependencyObject::SetValueCore
13 Windows_UI_Xaml!DirectUI::DependencyObject::SetValueCore
14 Windows_UI_Xaml!DirectUI::DependencyObject::SetValueByKnownIndex_int_
15 Windows_UI_Xaml!DirectUI::ComboBox::OnClose
16 Windows_UI_Xaml!DirectUI::ComboBox::OnIsDropDownOpenChanged
17 Windows_UI_Xaml!DirectUI::ComboBox::OnPropertyChanged2
18 Windows_UI_Xaml!DirectUI::DependencyObject::NotifyPropertyChanged
19 Windows_UI_Xaml!CDependencyObject::NotifyPropertyChanged
20 Windows_UI_Xaml!CDependencyObject::SetEffectiveValue
21 Windows_UI_Xaml!CDependencyObject::UpdateEffectiveValue
22 Windows_UI_Xaml!CDependencyObject::SetValue
23 Windows_UI_Xaml!CUIElement::SetValue
24 Windows_UI_Xaml!CFrameworkElement::SetValue
25 Windows_UI_Xaml!CControl::SetValue
26 Windows_UI_Xaml!CItemsControl::SetValue
27 Windows_UI_Xaml!DirectUI::DependencyObject::SetValueCore
28 Windows_UI_Xaml!DirectUI::DependencyObject::SetValueByKnownIndex_unsigned char_
29 Windows_UI_Xaml!DirectUI::ComboBox::FocusChanged
30 Windows_UI_Xaml!DirectUI::ComboBox::OnElementPopupChildLostFocus

Start with the provided crash stack, especially DirectUI::ComboBox::OnClose, OnIsDropDownOpenChanged, and Selector::OnSelectedIndexChanged, then locate the corresponding Calculator UI code. Determine which selector-derived control assigns the out-of-range index and add regression coverage that verifies the selected index remains valid.