bug#12872: bug#12867: 24.3.50; easy-to-repro crash involving mode line

[Drew Adams]

[from bug #12867]

> I mean a new option, an enhancement.
> 
> > > But we should first formulate the conditions under which this
> > > redisplay will be performed.
> > 
> > If we're talking about my use case then it is each time the 
> > current line changes.
> 
> Would it be good enough to redisplay whenever point moves, and let
> your code you run from :eval decide whether the text on the mode line
> needs to be changed?  I think this will be a more general solution.

Yes, it would be good enough.

But the advantage that I'm supposing %l has is that the line-counting is done in
C, as part of the display engine.

If my code had to check whether the line has changed then it would do that in
Lisp.  Not saying that's a big deal.  But it still looks to me like the %l
triggering is convenient.

Perhaps the option could handle both cases: the general point-change case and
the more particular line-change case, depending on the option value?

BTW, why would this be a user option, rather than just a variable that code can
bind?  The use case for users is not too clear to me.

I guess you want users to be able to turn off such triggering?  That is
something different from turning off redisplay caused by such triggering (of
course, inhibiting the triggering turns off its resulting redisplay also).

Anyway, I don't have much to say about what should be done for this enhancement.

> Done: bug #12872.  Let's continue there.

Thank you.