bug#22404: 25.1.50; Forcing `window-scroll-functions` to run.

[Keith David Bershatsky]

Thank you, Eli, for taking a look at this feature request.

The post-command-hook ("PCH") is only sufficient to return the correct 
window-start and window-end when no scrolling occurs.  If I attach a function 
to the PCH that places overlays, then I would need to know programmatically 
with 100% certainty whether the window-scroll-functions ("WSF") will be running 
one or more times subsequent to the PCH.  If the WSF will be running one or 
more times subsequent to the PCH, then I want to programmatically *prevent* 
overlays from being placed with the PCH -- i.e., so that they can instead be 
placed by the WSF.  If the WSF will run more than once, then I want to wait 
until the last time it runs to place my overlays -- i.e., because start/end 
have changed since the first time WSF ran.  E.g., point was partially visible 
and redisplay brings it into full view.

I was not able to come up with a 100% reliable test from the PCH that tells me 
whether WSF will be running one or more times subsequent thereto.  So, I am 
using the following work-around to force the WSF to run all the time.  The 
problem remaining is that I still need a reliable test to determine whether the 
WSF will be running more than once, so that I can delay my overlay function 
until the last call.  The feature request is essentially:  trigger WSF (similar 
to set-window-buffer), but there is no need to set-window-buffer and there is 
no need to run the window-configuration-change-hook.  If there is any way you 
can think of to programmatically know with 100% certainty whether the WSF will 
be running more than one time, your help would be greatly appreciated.

(setq scroll-conservatively 101)

(defun pch-fn ()
  (let ((window-configuration-change-hook nil))
    (set-window-buffer (selected-window) (current-buffer) 'keep-margins)))

(add-hook 'post-command-hook 'pch-fn nil 'local)

(defun wsf-fn (win start)
  (message "win: %s | start: %s | end: %s" win start (window-end win t)))

(add-hook 'window-scroll-functions 'wsf-fn nil 'local)

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At Tue, 19 Jan 2016 19:50:08 +0200,
Eli Zaretskii wrote:
> 
> * * *
> 
> Can you please tell more details about the problem you have?  I;ve
> read this description several times, and couldn't figure out why are
> you having problems.
> 
> For example, why can't you run your function from the
> post-command-hook directly?  If you want it to run after all the hook
> functions finished, you can use the APPEND argument to add-hook,
> right?
> 
> Or if the above somehow doesn't work, what about pre-command-hook?
> 
> And those are just 2 random thoughts that went through my head while I
> was reading your request.
> 
> Thanks.