ComboBox on a Flyout attached to an AppBarButton loses mouse input on 1607

Several developers have asked why a ComboBox on a Flyout attached to an AppBarButton ignores mouse clicks after upgrading their UWP apps to target build 14393 (Windows 10, version 1607 – also known as the Windows 10 Anniversary Update). The same Xaml worked fine unchanged when the app targeted build 10586 (Windows 10, version 1511). <tl;dr> Apps can…