I am not really a fan of heuristic solutions like this. As you say, in situations like in Reaper where the NSWindow outlives Baseview's NSView, this notification won't occur; and, in the reverse situation, where the host closes the containing NSWindow but retains a reference to Baseview's NSView and then reparents it to another window, this notification will result in premature destruction of the Baseview window.

There was a discussion on the Rust Audio Discord several months ago about changing Baseview's API contract so that windows may only be closed via an explicit call to WindowHandle::close() or Window::close(), which would resolve the refcount cycle issue in a more principled way. I'll write that up in more detail on #124.