Wow that is strange. Can you first save a copy of your prefs.json file here:
<user home folder>\Spine\prefs.json
If you can send this, that would help: contact@esotericsoftware.com
To fix it, first try resizing the Spine window. If that doesn't help, close Spine, delete your prefs.json file, then start Spine. Unfortunately you'll lose any settings you've changed in the Settings dialog.
