From: E Sabof <esabof@gmail.com>
To: Eli Zaretskii <eliz@gnu.org>
Cc: 18923@debbugs.gnu.org
Subject: bug#18923: Alternative scrolling model
Date: Sun, 02 Nov 2014 18:25:28 +0000 [thread overview]
Message-ID: <87oaspa293.fsf@gmail.com> (raw)
In-Reply-To: <838ujty6ns.fsf@gnu.org>
Eli Zaretskii <eliz@gnu.org> writes:
>> From: E Sabof <esabof@gmail.com>
>> Date: Sun, 02 Nov 2014 02:31:28 +0000
>>
>> > Sounds nice. Do you imagine it as a replacement for the existing
>> > scroll-up/down functions? Or rather (at least at first) as a separate
>> > package?
>> > Also, if something's missing for st-height to get more accurate
>> > measurements, I suggest you make it a bug report asking for that missing
>> > info/feature.
>>
>> I was mostly thinking the first.
>
> If this is intended as a replacement for the existing functionality,
> then it needs to support all the features that the current code
> supports. The list of those features should include at least the
> following:
>
> . the argument to the commands can be nil, which means "almost the
> full window", where "almost full" depends on the value of
> next-screen-context-lines
Easy to implement naively, and heuristics should probably be considered.
> . the auto-window-vscroll variable
I think the variable would no longer be useful with this approach.
> . the scroll-preserve-screen-position option
Ideally I'd measure the pixel distance from the top and try to restore
it, but alternatives can be considered if this affects performance.
> . signal an error at beginning and end of buffer, subject to the
> value of scroll-error-top-bottom
I think I already have the code that determines this case, I just need
to throw errors.
> . don't let point enter the scroll margin as result of scrolling
>
> . the window's old_point marker needs to be set after scrolling
I don't fully understand these, but they don't seem too complicated.
> There's also a bug when scrolling near the end of buffer: the result
> is that the cursor us shown on a line beyond EOB, which should never
> happen.
I've noticed this. The point flashes at a wrong position, and then goes
back to "legal" position. I thought this was a display engine bug.
>> The only potentially downside I can think of
>> is that it might be slower -- then again I'm just measuring line-heights, and
>> of these there is (at most) only one line that won't eventually be displayed.
>
> It is indeed much slower. I timed it on xdisp.c using Dmitry's
> scroll-up-benchmark function, and found this code to be 3 times slower
> than the current implementation. Turning off font-lock slashes about
> 40% of the benchmark time, so CC mode fontifications are not the main
> reason for the slowdown. If I compare the existing implementation
> with this one on xdisp.c with font-lock-mode turned off in both cases,
> this implementation is 16 times slower than what we have now.
>
> For the record, my timings are from an unoptimized build of a recent
> trunk, with your code byte-compiled.
>
> The general algorithm seems to be the same as in the current C
> implementation, so I doubt an ELisp implementation could match what we
> have in speed, let alone be faster.
>
> Now, I personally don't regard the scrolling command as something that
> needs to be lightning-fast (although others obviously do, see the
> on-going discussions on emacs-devel about that). But in this case, a
> single PageDown keypress takes close to a second to execute, which is
> slow enough to annoy. By contrast, the current implementation is
> almost instantaneous. (Again, this is in an unoptimized build; an
> optimized build should be about twice faster, but I think 0.4 sec for
> a single scroll might still annoy.)
I'm aware of some inefficiencies in my code. My guess probably doesn't
mean much, but maybe something along 2X could be achieved.
> Finally, it looks like this code forces Emacs to display every single
> screen it scrolls through, even when it cannot keep up. I guess
> that's due to the 'redisplay' calls. This makes the situation where
> someone leans on the PageDown key and then releases it very
> unpleasant: Emacs keeps scrolling for a long time, and I didn't find a
> way of interrupting that.
st-move probably shouldn't call (redisplay) at all. I'll see if I can convert my comments to reproducible bugs.
>> If it were to remain mostly elisp, it would need a reliable way to measure the
>> height of a line (essentially a `st-height' replacement), irrespective of
>> whether it's displayed.
>
> Did you try to use pos-visible-in-window-p? AFAIU, it gives you what
> you want, including for lines that are taller than the window.
>
>> It has also proven rather difficult to set the window
>> start "absolutely". I've documented my findings in `st-move'.
>
> Does this happen only when point is on an image? (The comments in
> st-move seem to talk only about this situation.) If so, could you
> show a simple test case to demonstrate the problem?
I think there are ~3 different bugs involved (vscroll not being displayed, vscroll being nullified, and the point-on-image cases). I'll see if I can describe
them better.
Evgeni
next prev parent reply other threads:[~2014-11-02 18:25 UTC|newest]
Thread overview: 21+ messages / expand[flat|nested] mbox.gz Atom feed top
2014-11-02 1:15 bug#18923: Alternative scrolling model E Sabof
[not found] ` <jwv61ey5qfb.fsf-monnier+emacsbugs@gnu.org>
[not found] ` <87vbmy9wdx.fsf@gmail.com>
2014-11-02 2:31 ` E Sabof
2014-11-02 15:16 ` Eli Zaretskii
2014-11-02 18:25 ` E Sabof [this message]
2014-11-02 18:36 ` Eli Zaretskii
[not found] ` <jwvioiy416g.fsf-monnier+emacsbugs@gnu.org>
[not found] ` <87sii1ahy9.fsf@gmail.com>
[not found] ` <jwvppd538tu.fsf-monnier+emacsbugs@gnu.org>
2014-11-02 23:10 ` E Sabof
2014-11-03 2:35 ` Stefan Monnier
2014-11-03 16:03 ` Eli Zaretskii
2014-11-03 19:02 ` E Sabof
2014-11-02 15:14 ` Eli Zaretskii
2014-11-02 15:56 ` Stefan Monnier
2014-11-02 16:06 ` Eli Zaretskii
2014-11-02 16:21 ` E Sabof
2014-11-02 16:31 ` Eli Zaretskii
2014-11-02 17:43 ` E Sabof
2014-11-02 18:22 ` Eli Zaretskii
2014-11-02 19:09 ` E Sabof
2014-11-02 19:29 ` Eli Zaretskii
2014-11-03 3:45 ` Eli Zaretskii
2019-11-04 9:14 ` Stefan Kangas
2022-04-22 12:16 ` Lars Ingebrigtsen
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