***Don’t have both a *.xaml User Control and a WinForm that contains the the control inside an ElementHost in it at the same time.***
Left Image: after restarting VS, you’ll get this message.
Right Image: When viewing your WinForm with the ElementHost that has the *.xaml reference. You can see that it is disabled.
All you have to do is close VS down and fire it back up again.
(Anyone else run into this issue? It’s annoying.)