bug#66732: tree-sitter fontification doesn't update multi-line syntax reliably

[Stefan Monnier]

>> [1] The chain of events if roughly: user types the last "/" -> redisplay 
>> -> fontify that character -> access parser -> parser reparses -> calls 
>> notifier.
>
> The way to mark region of text as not fontified is to set its
> 'fontified' property nil.

Indeed, but if you do it from within jit-lock it will not work as
expected because jit-lock will mark the chunk just processed with
`fontified` == t.

> Instead of a timer, did you try to use pre-redisplay-function?

Anything that runs "later" (i.e. after we exit `jit-lock-fontify-now`)
will work, but `pre-redisplay-function` means the display will only be
updated at the next redisplay, whereas with a timer we also force
a redisplay, thus shortening the time during which we display an
incompletely fontified chunk.

Jit-lock itself uses a similar timer for similar purposes.

We could make it slightly cheaper by exporting the `pending_funcalls` to
ELisp and then using it instead of a 0s timer.



        Stefan