First issues with the coco versions, I'm afraid.
In short: it's related to doing UI things from within Forth. The current
WindowServer is nagging at me, delaying GUI updates.
Working on it. No easy fix soon, sorry.
For now, don't use OSX 10.11 El Capitan with coco-sf, coco-vfx and
coco-i4. The issue doesn't appear in previous OSX versions at all, not
even a sign of problems ahead in Console.app logs AFAIK.
Or you can grit your teeth and bear the nuisance of a repl polluted by
warnings issued by the CGSUpdateManager (Core Graphics GUI Updater?).
It does this the first time you access the GUI, and then it keeps silent
it seems. You can turn the logging off in the Forth repl by executing
-STDERR. The GUI handling at times might look a bit flaky though. Good luck.
HTH
-r