bug#58980: 29.0.50; Mouse wheel scroll twitches up sometimes when I scroll down

bug#58980: 29.0.50; Mouse wheel scroll twitches up sometimes when I scroll down

[Mickey Petersen]

Note: I'm using the tree-sitter branch, but the issue also appears in 
the master
branch. Emacs 28.x works fine.

When I scroll down with my mouse wheel -- typically after an existing 
window has changed
buffers, such as after selecting (via keyboard or mouse) an Info manual 
node --
the scroll action sometimes (but not always) jerks upwards several
lines. I can then keep scrolling and it'll start scrolling down
again. This may sometimes happen after scrolling for a little while, and
sometimes not at all.

It's hard to reproduce; but the issue affects a baseline Emacs build (-Q or
-q) and only sometimes.

I had a look in the bug tracker for `scroll' and found nothing. I also
checked `M-x customize-changed' to look for likely culprits. I found
nothing that may cause this behaviour, beyond pixel scrolling, which 
seems like a smoking gun to me,
but it's off by default in emacs -Q.

1. Is it possible there are changes the scrolling code that is active even
when that mode is not?

2. Could there be some sort of variable that determines scroll direction 
that, somehow, gets confused?

Thanks.

Mickey.





In GNU Emacs 29.0.50 (build 2, x86_64-pc-linux-gnu, GTK+ Version
  3.24.20, cairo version 1.16.0) of 2022-10-28 built on mickey-work
Repository revision: 629f222ba2abf5a149c381e27c89e88963f7466d
Repository branch: feature/tree-sitter
Windowing system distributor 'The X.Org Foundation', version 11.0.12013000
System Description: Ubuntu 20.04.3 LTS

Configured using:
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bug#58980: 29.0.50; Mouse wheel scroll twitches up sometimes when I scroll down

[Eli Zaretskii]

> Date: Wed, 2 Nov 2022 16:20:40 +0000
> From: Mickey Petersen <mickey@fyeah.org>
> 
> When I scroll down with my mouse wheel -- typically after an existing 
> window has changed
> buffers, such as after selecting (via keyboard or mouse) an Info manual 
> node --
> the scroll action sometimes (but not always) jerks upwards several
> lines. I can then keep scrolling and it'll start scrolling down
> again. This may sometimes happen after scrolling for a little while, and
> sometimes not at all.
> 
> It's hard to reproduce; but the issue affects a baseline Emacs build (-Q or
> -q) and only sometimes.

Please provide some kind of reproduction recipe, even if it doesn't
happen every time.  When it does happen, type "C-h l", and post the
commands Emacs recorded that you were invoking immediately before the
problem.  Otherwise, it's very hard to do anything about the problem,
unless someone here can reproduce it (I couldn't).

> I had a look in the bug tracker for `scroll' and found nothing. I also
> checked `M-x customize-changed' to look for likely culprits. I found
> nothing that may cause this behaviour, beyond pixel scrolling, which 
> seems like a smoking gun to me,
> but it's off by default in emacs -Q.
> 
> 1. Is it possible there are changes the scrolling code that is active even
> when that mode is not?
> 
> 2. Could there be some sort of variable that determines scroll direction 
> that, somehow, gets confused?

Hard to answer these questions, but maybe Po Lu with have clues.

bug#58980: 29.0.50; Mouse wheel scroll twitches up sometimes when I scroll down

[Mickey Petersen]

On 03/11/2022 8:31 am, Eli Zaretskii wrote:
>> Date: Wed, 2 Nov 2022 16:20:40 +0000
>> From: Mickey Petersen <mickey@fyeah.org>
>>
>> When I scroll down with my mouse wheel -- typically after an existing
>> window has changed
>> buffers, such as after selecting (via keyboard or mouse) an Info manual
>> node --
>> the scroll action sometimes (but not always) jerks upwards several
>> lines. I can then keep scrolling and it'll start scrolling down
>> again. This may sometimes happen after scrolling for a little while, and
>> sometimes not at all.
>>
>> It's hard to reproduce; but the issue affects a baseline Emacs build (-Q or
>> -q) and only sometimes.
> Please provide some kind of reproduction recipe, even if it doesn't
> happen every time.  When it does happen, type "C-h l", and post the
> commands Emacs recorded that you were invoking immediately before the
> problem.  Otherwise, it's very hard to do anything about the problem,
> unless someone here can reproduce it (I couldn't).
Sure.

I was attempting to reproduce the issue -- by mouse-1 clicking in the 
already-open lossage buffer -- and then I began scrolling _down_ though 
the lossage buffer and it triggered the issue. It instead made me scroll 
up when I was in actual fact scrolling down.
The succession of wheel-up events eventually turned into wheel-down; 
despite the fact that I only scrolled down on my mouse wheel.

Lossage:

  C-h l               ;; view-lossage
  <down-mouse-1>      ;; mouse-drag-region <- I enter the lossage buffer
  <mouse-1>           ;; mouse-set-point
  <down-mouse-1>      ;; mouse-drag-region
  <mouse-1>           ;; mouse-set-point
  <wheel-up>          ;; mwheel-scroll <- I begin scrolling down on my 
physical mouse wheel
  <wheel-up>          ;; mwheel-scroll
  <wheel-up>          ;; mwheel-scroll
  <wheel-up>          ;; mwheel-scroll
  <wheel-up>          ;; mwheel-scroll
  <wheel-down>        ;; mwheel-scroll <- It corrects itself and 
actually starts scrolling down.
  <wheel-down>        ;; mwheel-scroll
  <wheel-down>        ;; mwheel-scroll
  <wheel-down>        ;; mwheel-scroll
  <double-wheel-down> ;; mwheel-scroll
  <triple-wheel-down> ;; mwheel-scroll
  <wheel-down>        ;; mwheel-scroll
  C-h l               ;; view-lossage <- I refresh the lossage.


I _do_, however, have a way that seems to reproduce the issue: when I 
scroll an inactive window and then go back to scrolling the active 
window the bug manifests itself more easily:

1. Ensure there are two windows minimum.
2. Select one.
3. Scroll the other down by hovering over it with your mouse.
4. Now attempt to scroll the active window. Mine will scroll up, and 
trigger "beginning of buffer" user errors if it's already at the top.

Seems there's some stateful behaviour that does or does not get carried 
over when the 'hovered window scroll' changes?


>> I had a look in the bug tracker for `scroll' and found nothing. I also
>> checked `M-x customize-changed' to look for likely culprits. I found
>> nothing that may cause this behaviour, beyond pixel scrolling, which
>> seems like a smoking gun to me,
>> but it's off by default in emacs -Q.
>>
>> 1. Is it possible there are changes the scrolling code that is active even
>> when that mode is not?
>>
>> 2. Could there be some sort of variable that determines scroll direction
>> that, somehow, gets confused?
> Hard to answer these questions, but maybe Po Lu with have clues.

bug#58980: 29.0.50; Mouse wheel scroll twitches up sometimes when I scroll down

[Eli Zaretskii]

> Date: Thu, 3 Nov 2022 09:46:46 +0000
> Cc: 58980@debbugs.gnu.org
> From: Mickey Petersen <mickey@fyeah.org>
> 
> 1. Ensure there are two windows minimum.
> 2. Select one.
> 3. Scroll the other down by hovering over it with your mouse.
> 4. Now attempt to scroll the active window. Mine will scroll up, and 
> trigger "beginning of buffer" user errors if it's already at the top.

Still not reproducible here, sorry.  But I'm not on X, so maybe this
is specific to X Windows.

bug#58980: 29.0.50; Mouse wheel scroll twitches up sometimes when I scroll down

[Po Lu via Bug reports for GNU Emacs, the Swiss army knife of text editors]

Eli Zaretskii <eliz@gnu.org> writes:

> Hard to answer these questions, but maybe Po Lu with have clues.

I think I can guess what this is: some problem with scroll valuator
tracking.

Mickye, does it still happen with a fresh build from master?  I removed
the valuator reset optimizations that were hitting X server bugs one or
two weeks ago.

bug#58980: 29.0.50; Mouse wheel scroll twitches up sometimes when I scroll down

[Po Lu via Bug reports for GNU Emacs, the Swiss army knife of text editors]

Mickey Petersen <mickey@fyeah.org> writes:

> I _do_, however, have a way that seems to reproduce the issue: when I
> scroll an inactive window and then go back to scrolling the active
> window the bug manifests itself more easily:
>
> 1. Ensure there are two windows minimum.

Windows, or frames?

> Seems there's some stateful behaviour that does or does not get
> carried over when the 'hovered window scroll' changes?

No, that shouldn't be it.  Emacs tracks scroll valuators on a
display-local basis, but only gets motion events for the valuator when
the mouse pointer is inside a frame.  As a result, scroll valuators are
reset upon crossing events, as by the time an XI_Enter event arrives,
the stored value can no longer be trusted.

So please answer the question above.

bug#58980: 29.0.50; Mouse wheel scroll twitches up sometimes when I scroll down

[Po Lu via Bug reports for GNU Emacs, the Swiss army knife of text editors]

Mickey Petersen <mickey@fyeah.org> writes:

> That did indeed seem to fix the issue, but unfortunately it did seem
> to introduce one minor regression:
>
> When you first scroll a window, it now takes two scrolls of my wheel
> to induce one scroll event in Emacs. After that, it works fine, and
> subsequent 'chains' of up or down scrolls work as expected.
> It's again 'reset' if I try to scroll a different, non-active, window;
> then I must scroll twice in a direction to get Emacs to register a
> scroll event.
>
> Could there be an off-by-one error somewhere?

Luckily, what I said earlier isn't it.  Please test again and see if the
problem is fixed now.

Thanks; BTW, please keep the bug tracker copied in by using "Reply to
All" (or its equivalent in your MUA.)

bug#58980: 29.0.50; Mouse wheel scroll twitches up sometimes when I scroll down

[Mickey Petersen]

On 04/11/2022 11:22 am, Po Lu wrote:
> Mickey Petersen <mickey@fyeah.org> writes:
>
>> That did indeed seem to fix the issue, but unfortunately it did seem
>> to introduce one minor regression:
>>
>> When you first scroll a window, it now takes two scrolls of my wheel
>> to induce one scroll event in Emacs. After that, it works fine, and
>> subsequent 'chains' of up or down scrolls work as expected.
>> It's again 'reset' if I try to scroll a different, non-active, window;
>> then I must scroll twice in a direction to get Emacs to register a
>> scroll event.
>>
>> Could there be an off-by-one error somewhere?
> Luckily, what I said earlier isn't it.  Please test again and see if the
> problem is fixed now.
I can confirm that this fix now resolves both the regression and the 
original issue.

I'll keep you posted if I discover any other oddities. (Pixel scroll 
precision mode also works fine, by the way!)

Thanks, Po lu.
>
> Thanks; BTW, please keep the bug tracker copied in by using "Reply to
> All" (or its equivalent in your MUA.)
Oops - thanks!

bug#58980: 29.0.50; Mouse wheel scroll twitches up sometimes when I scroll down

[Po Lu via Bug reports for GNU Emacs, the Swiss army knife of text editors]

Mickey Petersen <mickey@fyeah.org> writes:

> I can confirm that this fix now resolves both the regression and the
> original issue.
>
> I'll keep you posted if I discover any other oddities. (Pixel scroll
> precision mode also works fine, by the way!)

Thanks for testing.  Please be sure to report any future issues here, as
we want Emacs 29 to be released with wheel movement working everywhere,
including on odd hardware setups that I cannot test myself (my guess is
that you use a trackpad to move the pointer but a mouse wheel to scroll,
or have some integrated keyboard-trackpad device.)

With that, I'm closing this bug.

bug#58980: 29.0.50; Mouse wheel scroll twitches up sometimes when I scroll down

[Mickey Petersen]

On 04/11/2022 11:30 am, Po Lu wrote:
> Mickey Petersen <mickey@fyeah.org> writes:
>
>> I can confirm that this fix now resolves both the regression and the
>> original issue.
>>
>> I'll keep you posted if I discover any other oddities. (Pixel scroll
>> precision mode also works fine, by the way!)
> Thanks for testing.  Please be sure to report any future issues here, as
> we want Emacs 29 to be released with wheel movement working everywhere,
> including on odd hardware setups that I cannot test myself (my guess is
> that you use a trackpad to move the pointer but a mouse wheel to scroll,
> or have some integrated keyboard-trackpad device.)
Actually my setup is rather vanilla: desktop mouse with a 'clicky' 
scroll wheel -- as opposed to a smooth-scrolling one -- running Ubuntu 
in a VM. So the mouse hardware is definitely an emulated one as far as 
X/Linux is concerned.
>
> With that, I'm closing this bug.