We just upgraded to 2017r3 recently and I am trying out old projects on a macOS with Retina display...
What puzzles me is the fact that built-in Xojo examples are displayed fine on retina...but simple older projects just show ugly font renderings when running, but not inside IDE.
This screenshot shows on top the IDE version and below when running the application.
I can't seem to find any settings regarding retina support...though I suspect it should work out of the box with macOS elects like labels, lists, buttons and so on.
Edit (Paul): Used embeddable image URL