This may be very obvious in hindsight. When installing the new VS Mac, and creating an ASPNET Core project, I can't run it by clicking the run button in the top left.
I can run if I right click on the Web project, click 'Run With' then 'Custom Configuration'. Works great.
However, the run item is disabled and I can't figure any way to enable it: http://prntscr.com/f7rsgh