Re: The unwarranted scrolling assumption

[Lennart Borgman]

On Fri, Jun 18, 2010 at 5:22 PM, Stefan Monnier
<address@hidden> wrote:
>>> I already did, and I think that code is correct.  Obviously,
>>> clip_changed needs to be set whenever the narrowing changes.
>> Ah, finally. Good.
>> And why do you think that?
>
> because src/buffer.h says:
>
>  /* Set nonzero whenever the narrowing is changed in this buffer.  */
>  int clip_changed;
>
> "clipping" and "narrowing" are the same thing.  So, changing the
> narrowing should say that clipping has changed.  All it says is
> "narrowing has been changed at some point, don't forget to check it".
> So it's the redisplay's responsability to determine whether narrowing is
> indeed different from what it was during the last redisplay (and
> clip_changed is just a variable that can save us from performing this
> check).  If setting clip_changed to 1 in the main loop has some other
> effect than slowing things down, that means we have a bug in redisplay.


Yes, clipping and narrowing is the same things, but clip_changed is
just something the display engine uses to see whether a redisplay is
necessary.

Currently redisplay is mislead by narrow_to_region etc setting
clip_changed to 1. The problem as I understands it is that redisplay
has not saved enough information to know whether the clip_changed
should be reset to 0 after the narrowing routines have set it to 1.

My patch fixes this by letting redisplay have the whole responsibility
for clip_changed. There are of course other ways to do it, but from a
logic point that seems the best and I think it gives much cleaner
code.