bug#57207: 29.0.50; Fontification is slow after e7b5912b23 (Improvements to long lines handling)

[Gregory Heytings]

> > Stefan said (IIUC) that it could be useful for Lisp programs, so it is 
> > now possible to use it anywhere (but the docstring says that it should 
> > be done sparingly).
>
> Which means that the code you installed on the branch will "undo" such 
> narrowing done from Lisp as well, doesn't it?  That might not be what 
> the Lisp program which did that wanted.

I'm not sure I understand what you mean.  What the modeline should display 
is the state of the "user" narrowing, not of a narrowing decided by a Lisp 
function.  Changing the user narrowing from inside a hook function cannot 
be guaranteed to work, as far as I understand, as any other later hook 
function could change or undo that narrowing.

> It is better to do that higher, because not only those places in 
> decode_mode_spec may wish to access BEGV and ZV.  Even in the mode-line 
> display there are :eval and :when.

Okay, I'll see what I can do.

> > IMO we should just give it a try, it will always be possible to step 
> > back if it transpires that the cost is higher than the benefit.
>
> That's we are doing.  But we could also do it the other way around: fix 
> the places we know are problematic in Lisp, and then wait for enough 
> such problems to appear elsewhere.

Indeed.  As I said, I don't know which path is better.  My feeling (which 
could again be wrong) is that the current one is safer from a "responsive 
Emacs" viewpoint.