That’s because before the user types the final “/“, there isn’t a complete comment node in the parse tree. When the user types the “/“, we need to mark the whole block comment for refontification.

We actually have that, when the parser reparses, it’ll also compute the affected region, the region that changed during the last reparse, and it’ll call the “notifiers” with that region. We install a font-lock-notifier, which simply sets fontified text property to nil in that region, so redisplay would call jit-lock to fontify that region.

In our example, the region would be the block comment.

>> We might need to run (progn (force-parse) (update-ranges) (force-parse))
>> before jit-lock-fontify-now and sytax-ppss.
> 
> Why would we need that?

The first force-parse is just to make the parser reparse the latest buffer content, then we need to update ranges of any embedded code, then we would want to make the parser for the embedded language to reparse, if their range has changed. After this process, all the parsers has the up-to-date parse tree, and more importantly have called their notifiers, those notifiers should apply fontified text prop, and apply any syntax text prop.

Yuan