GNU bug report logs - #19684
25.0.50; `this-command` is set to `nil` before `window-scroll-functions` hook runs.

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From: help-debbugs <at> gnu.org (GNU bug Tracking System)
To: Keith David Bershatsky <esq <at> lawlist.com>
Subject: bug#19684: closed (Re: bug#19684: 25.0.50; `this-command` is set
 to `nil` before `window-scroll-functions` hook runs.)
Date: Tue, 01 Oct 2019 20:18:02 +0000

[Message part 1 (text/plain, inline)]

Your bug report

#19684: 25.0.50; `this-command` is set to `nil` before `window-scroll-functions` hook runs.

which was filed against the emacs package, has been closed.

The explanation is attached below, along with your original report.
If you require more details, please reply to 19684 <at> debbugs.gnu.org.

-- 
19684: http://debbugs.gnu.org/cgi/bugreport.cgi?bug=19684
GNU Bug Tracking System
Contact help-debbugs <at> gnu.org with problems

[Message part 2 (message/rfc822, inline)]

From: Stefan Kangas <stefan <at> marxist.se>
To: Stefan Monnier <monnier <at> iro.umontreal.ca>
Cc: Keith David Bershatsky <esq <at> lawlist.com>, 19684-done <at> debbugs.gnu.org
Subject: Re: bug#19684: 25.0.50; `this-command` is set to `nil` before
 `window-scroll-functions` hook runs.
Date: Tue, 1 Oct 2019 22:16:57 +0200

Stefan Monnier <monnier <at> IRO.UMontreal.CA> writes:

> tags 19684 notabug
> thanks
>
>> 1.  Expressly call `redisplay` at the tail end of the series of
>>     functions attached to the `post-command-hook`.
>>     Calling `redisplay` somehow prevents `this-command` from being
>>     reset to `nil` and the `window-scroll-functions` hook can then
>>     take advantage of the existing value of `this-command`.
>
> It's not that calling `redisplay' prevents it.  It's that the normal
> redisplay is run later (potentially much later if there are other things
> pending), at a point which is not directly connected with the command,
> hence this-command is not set any more because we're really not in the
> process of running that command any more.

This was already tagged notabug; I'm now also closing it.

Best regards,
Stefan Kangas

[Message part 3 (message/rfc822, inline)]

From: Keith David Bershatsky <esq <at> lawlist.com>
To: bug-gnu-emacs <at> gnu.org
Subject: 25.0.50;
 `this-command` is set to `nil` before `window-scroll-functions` hook
 runs.
Date: Sat, 24 Jan 2015 23:19:48 -0800

To the extent that a user wishes to use the value of `this-command` within a function attached to the `window-scroll-functions` hook, a workaround is needed because the default value is `nil`.  In other words, something at the C source code level is setting the value to `nil` before the `window-scroll-functions` hook runs.

There are two (2) workarounds that I am aware of.

1.  Expressly call `redisplay` at the tail end of the series of functions attached to the `post-command-hook`.  Calling `redisplay` somehow prevents `this-command` from being reset to `nil` and the `window-scroll-functions` hook can then take advantage of the existing value of `this-command`.

2.  Create a separate variable -- e.g., `my-this-command` and set the value within a function attached to the `post-command-hook`.  Like so:  (setq my-this-command this-command).  And, then use the value of `my-this-command` within the function attached to the `window-scroll-functions` hook.

The most reliable test I have discovered to pinpoint when the `window-scroll-functions` hook will activate is by having the following code within a function attached to the `post-command-hook`:  (pos-visible-in-window-p (point) (get-buffer-window (current-buffer) (selected-frame))).  If the result is `t`, then the `window-scroll-functions` hook will be activating.

I do not see a visual difference between either of the two workarounds mentioned above, even though I would expect to see a flicker on the screen with the first workaround.  However, there is no flicker.  The second workaround is probably a safer bet.

Although this bug report has been submitted with Emacs Trunk from October 1, 2014; the behavior is the same with Emacs Trunk built on January 1, 2015.

Thanks,

Keith

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In GNU Emacs 25.0.50.1 (x86_64-apple-darwin10.8.0, NS appkit-1038.36 Version 10.6.8 (Build 10K549))
 of 2014-10-01 on MP.local
Repository revision: 117996 dmantipov <at> yandex.ru-20141001132108-zdsxru2390mqyjlu
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