This is actually a rather old bug, but I don't know if it has been reported. I've found that when working on fonts with large and complex lookups, once it becomes necessary to use the useExtension directive in your lookups, the opentype preview pane stops processing contextual substitutions (simple substitutions continue to work).
This is a somewhat aggravating bug since it means that for more complex fonts one cannot test changes to opentype code without first generating and then installing the font, which sometimes requires clearing caches, restarting, etc. It would be much more convenient if tweaks to contextual substitutions could be tested directly within the application.
André
