How to show a modal window in .NET MAUI 11

Two different things get called a modal window in .NET MAUI 11. PushModalAsync gives you a modal page on every platform. A real OS window that disables its owner has no MAUI API at all, so here is the WinUI OverlappedPresenter.IsModal plus Win32 owner-handle interop that actually works on Windows, and what to do on Mac Catalyst instead.