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

[Keith David Bershatsky]

As a feature request, please consider adding an alternative method to force the 
`window-scroll-functions` hook to run -- but NOT more than it would normally 
run. [In other words, it shouldn't run 3 times.]  I only found 
`(set-window-buffer (selected-window) (current-buffer))` as a viable means to 
achieve this goal.  `run-window-scroll-functions` runs immediately and is not a 
viable substitute for this feature request.  Some users may not wish to 
`set-window-buffer` or run the `window-configuration-change-hook` to achieve 
this feature request.

In my particular use case, I want to run a custom function only one time per 
command loop that is dependent upon the new `window-start` and `window-end`, 
which is only reliably available to the user after the `post-command-hook` has 
run its course.  My function is too costly time-wise to run more than once each 
command loop.  I have tried, to no avail, to come up with a test to ascertain 
with 100% accuracy (from the `post-command-hook`) to guess whether the 
`window-scroll-functions` hook will run and/or whether it will run more than 
one time (e.g., when cursor/point is partially visible).  I played around with 
`(pos-visible-in-window-p nil nil t)` and whether it returned a length of 0, 2 
or 6 -- however, from the `post-command-hook`, that is insufficient to 
ascertain with 100% accuracy whether the `window-scroll-functions` hook will 
run once or twice or not at all.  The solution, I believe, is to force the 
`window-scroll-functions` hook to run during every redisplay -- but not more 
than it would normally run.

In other words, I want the `window-scroll-functions` hook to run every command 
loop when my custom minor-mode is active -- or twice if point/cursor is only 
partially visible (at the bottom of the window) and needs to be moved up a 
smidgen by redisplay.

Background:  my custom function is a cross-hairs created with overlays that 
works with `word-wrap` -- the vertical line is XPM that matches the character 
and color underneath.

Thanks,

Keith

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In GNU Emacs 25.1.50.1 (x86_64-apple-darwin10.8.0, NS appkit-1038.36 Version 
10.6.8 (Build 10K549))
 of 2016-01-03 built on server.local
Repository revision: e94b1799d4f4c57266bdbc4801b26fe0121b7c7a
Windowing system distributor 'Apple', version 10.3.1038
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