I don’t know if it’s a known issue, but sometimes the entire Studio interface becomes disabled (it is all grayed out, and cannot click on any control any more).
If it happens in the middle of editing something (especially a screen), in addition to become unresponsive, I noted that the “OK”, “Close” etc. buttons in the upper right corner disappear.
The only way to recover from this state is to refresh the browser, losing any unsaved change…
MacBook Pro with macOS 10.11.6
Browser Safari 10.1.1