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bug#18923: Alternative scrolling model
From: |
Eli Zaretskii |
Subject: |
bug#18923: Alternative scrolling model |
Date: |
Sun, 02 Nov 2014 17:16:23 +0200 |
> 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
. the auto-window-vscroll variable
. the scroll-preserve-screen-position option
. signal an error at beginning and end of buffer, subject to the
value of scroll-error-top-bottom
. don't let point enter the scroll margin as result of scrolling
. the window's old_point marker needs to be set after scrolling
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.
> 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.)
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.
> 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?
bug#18923: Alternative scrolling model, Eli Zaretskii, 2014/11/02