Thanks for working on this.

The deeper problem is, if there is some regex-based-font-lock face in
> region A (applied when Emacs fontified region A), eg, highlighted TODO
> keywords in a docstring, they will be overwritten by the string face, if we
> just apply string face to the whole string and trigger redisplay.
>

I still need to get familiar with the parts of the code involved. But can
we avoid this (potentially) expensive re-fontification by keeping a list of
text properties/fonts applied to a region? Because if the region was not
changed, aside from adding/removing comments, then it would not need to be
re-parsed.


> What I ended up doing is to set jit-lock-context-unfontify-pos to the
> beginning of the string node (aka beginning of region A). Then in a timer
> jit-lock-context will refontify everything after that position. And I have
> some measure to break possible infinite recursion (fontify region -> set
> jit-lock-context-unfontify-pos -> cause refontification -> fontify region
> -> …).
>

I still need to look at your fix more carefully as a similar issue exists
for sh-mode and heredoc strings.

-- 
João Paulo L. de Carvalho
Ph.D Computer Science |  IC-UNICAMP | Campinas , SP - Brazil
Postdoctoral Research Fellow | University of Alberta | Edmonton, AB - Canada
joao.carvalho@ic.unicamp.br
joao.carvalho@ualberta.ca