After doing the research on CoreWindow, I realized we could also make this Send + Sync (like #196), but under different constraints. CoreWindow itself is !Send and there is really nothing we can change here. But WinRT offers a mechanism similar to MacOS's libdispatch, by wrapping the type into a IAgileReference. The wrapper is Send and it can be used to "marshal" function calls to the right thread.

So we have to options:

• Force WinRtWindowHandle to be a IAgileReference<CoreWindow> instead of CoreWindow and make it Send + Sync.
• Introduce a new WinRtAgileWindowHandle that is a IAgileReference<CoreWindow>.

Considering there is no overhead for IAgileReference, apart from the cost of creation and resolving, we might want to consider just making it the default WinRtWindowHandle.