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

[Keith David Bershatsky]

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@yandex.ru-20141001132108-zdsxru2390mqyjlu
Windowing system distributor `Apple', version 10.3.1038
Configured using:
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