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

[Keith David Bershatsky]

In reading the comments in `xdisp.c`, there is a lot of mention about checking 
to see if the function that ran changed `window-start` and whether the cursor 
is still visible.  In my own tests over the past few days, I created something 
just like `post-redisplay-hook` that runs towards the end of redisplay.  I felt 
that my own feeble attempt to create the equivalent of the 
`post-redisplay-hook` had no checking built-in (due to lack knowledge / 
experience on my part).

So, I love the idea of a `post-redisplay-hook`, but think there probably needs 
to be some type of check to see whether the function that ran from it did some 
things that require certain recalculations -- e.g., maybe cursor was moved and 
is no longer partially or fully visible (there is even a setting I discovered 
where a user can decide that a partially visible cursor is just fine), maybe 
some text was inserted prior to window-start (which means the prior value is 
outdated), etc.

Sorry that I didn't understand your previous comments about the 
`pre-command-hook` potentially being a substitute for the contemplated 
`post-redisplay-hook`.  Michael is correct -- the goal is generally to have the 
screen looking perfectly once redisplay concludes and the `pre-command-hook` 
(after the fact) can't accomplish that goal.  When switching windows, the 
`pre-command-hook` will also have the wrong window in mind -- the correct 
window/buffer needs to be determined after the main command finishes.

Attached is an example of what I have been using for the past few days while I 
continue my ongoing tinkering -- it creates a new animal just like 
`window-scroll-functions` but with a different name.

Keith

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At Fri, 29 Jan 2016 16:37:16 +0200,
Eli Zaretskii wrote:
> 
> > From: Michael Heerdegen <michael_heerdegen@web.de>
> > Cc: Keith David Bershatsky <esq@lawlist.com>,  22404@debbugs.gnu.org
> > Date: Fri, 29 Jan 2016 13:00:42 +0100
> > 
> > Anyway, using pre-command-hook is too late for us: it's not executed
> > unless the next input arrives, so you have to hit a key until
> > decorations or whatever get updated.
> 
> Then maybe we need a post-redisplay-hook.

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