bug#50269: 27.2; Request: use GTK continuous scroll events for smooth touchpad scrolling

bug#50269: 27.2; Request: use GTK continuous scroll events for smooth touchpad scrolling

[Alexander Huntley]

When using a laptop touchpad, native GTK applications use continuous
scroll events to give much smoother scrolling, whereas Emacs simply
emulates the discrete scrolling events of a mouse scroll wheel. This
makes touchpad scrolling under Emacs feel much less natural than many
other Linux applications.

Would it be possible to expose these events to elisp and hence get nice
pixel-perfect scrolling in Emacs?

Thanks!
Alex Huntley


In GNU Emacs 27.2 (build 1, x86_64-redhat-linux-gnu, GTK+ Version 
3.24.30, cairo version 1.17.4)
  of 2021-08-07 built on buildvm-x86-29.iad2.fedoraproject.org
Windowing system distributor 'The X.Org Foundation', version 11.0.12101002
System Description: Fedora 34 (Workstation Edition)

bug#50269: 27.2; Request: use GTK continuous scroll events for smooth touchpad scrolling

[Eli Zaretskii]

> From: Alexander Huntley <huntley.alexander@gmail.com>
> Date: Mon, 30 Aug 2021 14:35:19 +0100
> 
> When using a laptop touchpad, native GTK applications use continuous
> scroll events to give much smoother scrolling, whereas Emacs simply
> emulates the discrete scrolling events of a mouse scroll wheel. This
> makes touchpad scrolling under Emacs feel much less natural than many
> other Linux applications.
> 
> Would it be possible to expose these events to elisp and hence get nice
> pixel-perfect scrolling in Emacs?

Did you try "M-x pixel-scroll-mode RET"?  It isn't perfect, but maybe
it's "good enough" for your needs?

bug#50269: 27.2; Request: use GTK continuous scroll events for smooth touchpad scrolling

[Alexander Huntley]

> Did you try "M-x pixel-scroll-mode RET"?  It isn't perfect, but maybe
> it's "good enough" for your needs?

Yes I have, as well as good-scroll.el; they're both great improvements,
especially for a mouse, but still don't quite feel as good as the scrolling
in Firefox or Gedit.

Obviously it's not a deal-breaker, but it would make Emacs feel more 
polished
and modern.

If I wanted to work on this, do you know where I could start? I'm currently
unfamiliar with the Emacs codebase.

bug#50269: 27.2; Request: use GTK continuous scroll events for smooth touchpad scrolling

[Eli Zaretskii]

> From: Alexander Huntley <huntley.alexander@gmail.com>
> Cc: 50269@debbugs.gnu.org
> Date: Mon, 30 Aug 2021 19:37:26 +0100
> 
> > Did you try "M-x pixel-scroll-mode RET"?  It isn't perfect, but maybe
> > it's "good enough" for your needs?
> 
> Yes I have, as well as good-scroll.el; they're both great improvements,
> especially for a mouse, but still don't quite feel as good as the scrolling
> in Firefox or Gedit.
> 
> Obviously it's not a deal-breaker, but it would make Emacs feel more 
> polished
> and modern.

Then I don't think I understand what you'd like Emacs to do in this
case, and how would that be different from pixel-scroll-mode.  Please
tell more.

bug#50269: 27.2; Request: use GTK continuous scroll events for smooth touchpad scrolling

[Alexander Huntley]

> Then I don't think I understand what you'd like Emacs to do in this
> case, and how would that be different from pixel-scroll-mode.  Please
> tell more.
Modern touchpads offer more precision than scroll wheels, so it is possible
(e.g. in Firefox, using libinput drivers) to make the view scroll by
just 1 or 2 pixels using small movements. As a result, the scrolling
corresponds closely to the hand movement on the trackpad. This feels
good (like scrolling on a touchscreen).

Emacs (because it ignores this higher-precision scrolling data) basically
quantizes the available scroll positions, which ought to be continuous.
This makes scrolling on Emacs feel more abrupt than in other apps.

pixel-scroll-mode does not fix this; it just provides an animation between
scroll positions. The available positions remain quantized at line
boundaries.

bug#50269: 27.2; Request: use GTK continuous scroll events for smooth touchpad scrolling

[Eli Zaretskii]

> From: Alexander Huntley <huntley.alexander@gmail.com>
> Cc: 50269@debbugs.gnu.org
> Date: Mon, 30 Aug 2021 20:02:00 +0100
> 
> > Then I don't think I understand what you'd like Emacs to do in this
> > case, and how would that be different from pixel-scroll-mode.  Please
> > tell more.
> Modern touchpads offer more precision than scroll wheels, so it is possible
> (e.g. in Firefox, using libinput drivers) to make the view scroll by
> just 1 or 2 pixels using small movements. As a result, the scrolling
> corresponds closely to the hand movement on the trackpad. This feels
> good (like scrolling on a touchscreen).
> 
> Emacs (because it ignores this higher-precision scrolling data) basically
> quantizes the available scroll positions, which ought to be continuous.
> This makes scrolling on Emacs feel more abrupt than in other apps.
> 
> pixel-scroll-mode does not fix this; it just provides an animation between
> scroll positions. The available positions remain quantized at line
> boundaries.

We are probably having communication difficulties due to terminology
you are using.  pixel-scroll-mode doesn't work on line granularity, it
actually shifts the display one pixel at a time.  If you scroll by
enough pixels so that the sum total of those pixels amounts to one
line, pixel-scroll-mode resets the display shift offset to zero and
scrolls the display by one full line, then it keeps shifting one pixel
at a time.

Given this description of how pixel-scroll-mode works, what exactly
would you like to change?

Or maybe looking at this from a different angle: how does the behavior
you'd like to see differ from what pixel-scroll-mode produces?

bug#50269: 27.2; Request: use GTK continuous scroll events for smooth touchpad scrolling

[Alexander Huntley]

> We are probably having communication difficulties due to terminology
> you are using.  pixel-scroll-mode doesn't work on line granularity, it
> actually shifts the display one pixel at a time.  If you scroll by
> enough pixels so that the sum total of those pixels amounts to one
> line, pixel-scroll-mode resets the display shift offset to zero and
> scrolls the display by one full line, then it keeps shifting one pixel
> at a time.
>
> Given this description of how pixel-scroll-mode works, what exactly
> would you like to change?
>
> Or maybe looking at this from a different angle: how does the behavior
> you'd like to see differ from what pixel-scroll-mode produces?
pixel-scroll-mode may shift the display one pixel at a time, but it
also "snaps" the display to certain larger intervals, (integer numbers
of lines).

This snapping makes the scrolling jerkier than it needs to be on
touchpads.

If we could use the more precise/frequent data actually coming from
the touchpad, then the user could control the scrolling more precisely:
instead of scrolling by n lines, the user could scroll by as little
as a single pixel at a time.

bug#50269: 27.2; Request: use GTK continuous scroll events for smooth touchpad scrolling

[Eli Zaretskii]

> From: Alexander Huntley <huntley.alexander@gmail.com>
> Cc: 50269@debbugs.gnu.org
> Date: Mon, 30 Aug 2021 22:26:45 +0100
> 
> pixel-scroll-mode may shift the display one pixel at a time, but it
> also "snaps" the display to certain larger intervals, (integer numbers
> of lines).

I don't think I follow: what is this "snapping" you are alluding to
here?  Can you tell me how to reproduce this snapping, so I could
understand better the problem you have in mind?

> If we could use the more precise/frequent data actually coming from
> the touchpad, then the user could control the scrolling more precisely:
> instead of scrolling by n lines, the user could scroll by as little
> as a single pixel at a time.

The scrolling, whether pixel-wise or not, is performed by the Emacs
display engine; the scroll commands themselves just tell the display
engine where in buffer text to start the display of a window, and with
what "shifting offset" in pixels to display that text.  So I don't see
how using some different/more precise data will help, if all that data
tells Emacs is by how many pixels to scroll, because the actual
scrolling will still be performed by the same display engine.  If the
display engine is unable to scroll smoothly, for some reason, using
different data source to compute how many pixels to scroll will not
help.

So we must first understand what is wrong with pixel-scroll-mode,
because all it does with the "normal" (a.k.a. "imprecise") scroll
commands coming from the touchpad is compute how many pixels to
scroll, and the default is AFAIR to scroll a small number of pixels
per wheel notch (you can customize it to make it exactly 1 pixel).

Therefore, I hope you could help us understand what is wrong with the
pixel-wise scrolling in the display engine, as it is used by
pixel-scroll-mode.  TIA.

bug#50269: 27.2; Request: use GTK continuous scroll events for smooth touchpad scrolling

[Kévin Le Gouguec]

Eli Zaretskii <eliz@gnu.org> writes:

>> From: Alexander Huntley <huntley.alexander@gmail.com>
>> Cc: 50269@debbugs.gnu.org
>> Date: Mon, 30 Aug 2021 22:26:45 +0100
>> 
>> pixel-scroll-mode may shift the display one pixel at a time, but it
>> also "snaps" the display to certain larger intervals, (integer numbers
>> of lines).
>
> I don't think I follow: what is this "snapping" you are alluding to
> here?  Can you tell me how to reproduce this snapping, so I could
> understand better the problem you have in mind?

With pixel-scroll-mode on, when pixel-resolution-fine-flag is nil (the
default), I cannot get Emacs to scroll up by "just a few pixels",
however small my movement on the touchpad.  Emacs always waits for
"enough movement" to scroll up a complete line, although the *animation*
is indeed done pixel-by-pixel (as opposed to line-by-line when
pixel-scroll-mode is off).

On e.g. Mousepad (XFCE's text editor), if I move "just a bit" on the
touchpad, the display moves by "just a few pixels", i.e. less than a
full line.  FWIW setting pixel-resolution-fine-flag to 1 seems to allow
Emacs to do the same.

(I hope I understood Alexander's problem correctly; apologies for the
noise if not)

bug#50269: 27.2; Request: use GTK continuous scroll events for smooth touchpad scrolling

[Eli Zaretskii]

> From: Kévin Le Gouguec <kevin.legouguec@gmail.com>
> Cc: Alexander Huntley <huntley.alexander@gmail.com>,  50269@debbugs.gnu.org
> Date: Tue, 31 Aug 2021 18:33:13 +0200
> 
> With pixel-scroll-mode on, when pixel-resolution-fine-flag is nil (the
> default), I cannot get Emacs to scroll up by "just a few pixels",
> however small my movement on the touchpad.  Emacs always waits for
> "enough movement" to scroll up a complete line, although the *animation*
> is indeed done pixel-by-pixel (as opposed to line-by-line when
> pixel-scroll-mode is off).
> 
> On e.g. Mousepad (XFCE's text editor), if I move "just a bit" on the
> touchpad, the display moves by "just a few pixels", i.e. less than a
> full line.  FWIW setting pixel-resolution-fine-flag to 1 seems to allow
> Emacs to do the same.

So if pixel-resolution-fine-flag is set, the behavior is the expected
one, and we can conclude that Emacs passes this test?

bug#50269: 27.2; Request: use GTK continuous scroll events for smooth touchpad scrolling

[Alexander Huntley]

>> With pixel-scroll-mode on, when pixel-resolution-fine-flag is nil (the
>> default), I cannot get Emacs to scroll up by "just a few pixels",
>> however small my movement on the touchpad.  Emacs always waits for
>> "enough movement" to scroll up a complete line, although the *animation*
>> is indeed done pixel-by-pixel (as opposed to line-by-line when
>> pixel-scroll-mode is off).
>>
>> On e.g. Mousepad (XFCE's text editor), if I move "just a bit" on the
>> touchpad, the display moves by "just a few pixels", i.e. less than a
>> full line.  FWIW setting pixel-resolution-fine-flag to 1 seems to allow
>> Emacs to do the same.
> So if pixel-resolution-fine-flag is set, the behavior is the expected
> one, and we can conclude that Emacs passes this test?

If the flag is set to 1, then yes the scrolling works as desired, but only
when scrolling with the touchpad slowly (although perhaps it is scrolling
by 2 or 3 pixels instead of only 1).

When scrolling quickly, Emacs scrolls abruptly by many lines at a
time, which is not what we want. I'm sure this is possible to disable.

But now scrolling with a mouse wheel also only causes the view to shift by
1 pixel per "wheel click".

IMO the best way to solve this would be to use GTK scrolling events. They
give the correct magnitude of the scroll event (which in touchpads is
variable and much smaller than for mouse wheels). Hence Emacs would not
have to distinguish between mouse vs touchpad scrolling; it would all
be handled by GTK.

bug#50269: 27.2; Request: use GTK continuous scroll events for smooth touchpad scrolling

[Eli Zaretskii]

> From: Alexander Huntley <huntley.alexander@gmail.com>
> Cc: Kévin Le Gouguec <kevin.legouguec@gmail.com>,
>  50269@debbugs.gnu.org
> Date: Wed, 1 Sep 2021 13:57:46 +0100
> 
> > So if pixel-resolution-fine-flag is set, the behavior is the expected
> > one, and we can conclude that Emacs passes this test?
> 
> If the flag is set to 1, then yes the scrolling works as desired, but only
> when scrolling with the touchpad slowly (although perhaps it is scrolling
> by 2 or 3 pixels instead of only 1).

That's good to hear, thanks.

> When scrolling quickly, Emacs scrolls abruptly by many lines at a
> time, which is not what we want. I'm sure this is possible to disable.

I'm guessing that the touchpad has some customization control, which
could control how it reports fast scrolling to applications.  Emacs
has similar controls in the variables mouse-wheel-scroll-amount and
mouse-wheel-progressive-speed.  I suggest to try playing with these
Emacs options and perhaps also with your touchpad controls, and try to
find the configuration that supports fast scrolling as well.

> IMO the best way to solve this would be to use GTK scrolling events. They
> give the correct magnitude of the scroll event (which in touchpads is
> variable and much smaller than for mouse wheels). Hence Emacs would not
> have to distinguish between mouse vs touchpad scrolling; it would all
> be handled by GTK.

AFAIU, this can only be possible in the upcoming PGTK configuration of
Emacs, if at all.  But I urge you to try customizing the options I
mentioned above, because perhaps you could have what you want much
sooner and easier than by waiting for PGTK to hit the streets.

bug#50269: 27.2; Request: use GTK continuous scroll events for smooth touchpad scrolling

[Alan Third]

On Mon, Aug 30, 2021 at 02:35:19PM +0100, Alexander Huntley wrote:
> When using a laptop touchpad, native GTK applications use continuous
> scroll events to give much smoother scrolling, whereas Emacs simply
> emulates the discrete scrolling events of a mouse scroll wheel. This
> makes touchpad scrolling under Emacs feel much less natural than many
> other Linux applications.
> 
> Would it be possible to expose these events to elisp and hence get nice
> pixel-perfect scrolling in Emacs?

It's not practical to do in the current X/GTK terminal code. Emacs
doesn't actually use the GTK event system, so it would have to be
added from scratch.

Once PGTK is merged it should be able to provide this sort of thing
much more easily.
-- 
Alan Third

bug#50269: 27.2; Request: use GTK continuous scroll events for smooth touchpad scrolling

[Alexander Huntley]

> It's not practical to do in the current X/GTK terminal code. Emacs
> doesn't actually use the GTK event system, so it would have to be
> added from scratch.
>
> Once PGTK is merged it should be able to provide this sort of thing
> much more easily.
Sounds promising! Is there any idea when that will happen?

bug#50269: 27.2; Request: use GTK continuous scroll events for smooth touchpad scrolling

[Alan Third]

On Mon, Aug 30, 2021 at 10:36:37PM +0100, Alexander Huntley wrote:
> 
> > It's not practical to do in the current X/GTK terminal code. Emacs
> > doesn't actually use the GTK event system, so it would have to be
> > added from scratch.
> > 
> > Once PGTK is merged it should be able to provide this sort of thing
> > much more easily.
> Sounds promising! Is there any idea when that will happen?

I believe the hope is it have it in for Emacs 28, but I don't actually
know how that's going.
-- 
Alan Third

bug#50269: 27.2; Request: use GTK continuous scroll events for smooth touchpad scrolling

[Lars Ingebrigtsen]

Alexander Huntley <huntley.alexander@gmail.com> writes:

> When using a laptop touchpad, native GTK applications use continuous
> scroll events to give much smoother scrolling, whereas Emacs simply
> emulates the discrete scrolling events of a mouse scroll wheel. This
> makes touchpad scrolling under Emacs feel much less natural than many
> other Linux applications.
>
> Would it be possible to expose these events to elisp and hence get nice
> pixel-perfect scrolling in Emacs?

(I'm going through old bug reports that unfortunately weren't resolved
at the time.)

There's been a great deal of work done in this area over the last few
months -- perhaps you could try the current "master" branch of Emacs and
see whether things work well for you there?

bug#50269: 27.2; Request: use GTK continuous scroll events for smooth touchpad scrolling

[Lars Ingebrigtsen]

Lars Ingebrigtsen <larsi@gnus.org> writes:

> There's been a great deal of work done in this area over the last few
> months -- perhaps you could try the current "master" branch of Emacs and
> see whether things work well for you there?

More information was requested, but no response was given within a
month, so I'm closing this bug report.  If the problem still exists,
please respond to this email and we'll reopen the bug report.