Sorry you are having trouble, especially right off the bat. I agree having to unplug all your monitors is definitely not how it should work.
There are indeed other Mac users using multiple monitors, including developers on our team. This is the first report we've had about this problem on macOS. Note there is a recent issue with some multi-monitor setups on Windows, but it is unrelated to your issue as the Windows and macOS windowing systems are completely different.
Unlike most other desktop apps, Spine uses OpenGL, like a game would, so it is closer to the metal and interfaces directly with your graphics drivers. Due to this, it's worth checking you have the latest graphics drivers.
We have had some problems on macOS with some other software interfering with Spine when run at the same time, such as Finder enhancements, Default Folder X, Rescue Time, or display sharing software like Display Link, Duet Display, or Twomon. Any chance you have something like that running? Obviously it would be better if there was not such a conflict, but it may make for an easier workaround for now. It may be that Spine can be run if the conflicting software is closed, but can be opened after Spine has started.
We have worked with the developers of one of the conflicting programs, but it ended with both of us at a loss as to why it happens. We plan to replace Spine's windowing system in the future and expect for this problem to be resolved at that time.
If you aren't able to find conflicting software, it may help if we can find more information. The first place to check is your spine.log file at ~/Library/Application Support/Spine/spine.log. It is rewritten each time you run Spine, so you'll need to run Spine with your monitors attached, then check the file.
You can also run the macOS Console app, click Clear Display, run Spine with the monitors attached, then click File > Save a Copy As and post it here, or email to us at: contact@esotericsoftware.com
Does the problem still occur if only a single monitor is attached? If so, can you say what brand and model is that monitor? Is it a high DPI (eg Retina) monitor? I'm looking for clues as to why we don't see the same problem.
