`windows-reactor`: AppContext::show_menu_at flyout cannot be shown over taskbar

[riverar]

MenuFlyout (used by AppContext::show_menu_at) is unfortunately restricted to the work area (https://github.com/microsoft/microsoft-ui-xaml/blob/main/dxaml/xcp/dxaml/lib/DXamlCore.cpp#L3843-L3860), likely an old "UWP" artifact. This means showing a flyout over a notification area icon is not possible as written.

MUX issue created: microsoft/microsoft-ui-xaml#11945

TBD on workarounds.