When changing focus away from game (e.g. alt-tab, clicking a notification, minimizing) while in full screen mode running any 1.13 snapshot or pre-release, the screen freezes.  All controls and sounds function normally, but the screen will remain static until the game is force closed.  This affects 1.13 versions 100% of the time.  Occasionally 1.12.2 will experience the same problem, or the screen will unfreeze after a few seconds.