It's a problem with Flash, High Sierra, and Stencyl 3.4. It's also an issue on some flavors of Linux. I'm not aware of any workaround, though than downgrading Mac OS or upgrading to Stencyl 3.5 (which is in beta right now, so only available to subscribers).
You can still test your game with the other options: HTML5, desktop, mobile, etc., so the simplest thing to do while you're in development would be just that.