bug#22637: 25.1.50; `mode-line` face `:height` incompatible with `scroll-conservatively 101`.

[Eli Zaretskii <eliz@gnu.org>]

> Date:  Mon, 15 Feb 2016 18:51:26 -0800
> From:  Keith David Bershatsky <esq@lawlist.com>
> Cc:  22637@debbugs.gnu.org
> 
> The same situation mentioned in my email earlier today (02/15/2016) is also present in the first test case that we looked at for #22637.   I.e., after an insertion in the first test case, the `window-scroll-functions` hook never fires after `window-start` is corrected.  So a user seeking to use the `window-scroll-functions` hook following a large insertion (e.g., a large yank, and so forth) is out of luck in that situation in terms of using a correct `window-start`.  I have already dealt with it for the `window-start-end-hook` by forcing a call every command loop thereby bypassing "optimization 3"; however, the WSF has no fix at this time.

The reason window-scroll-functions aren't run in both of these test
cases is that what happens there is not considered "scrolling".
Scrolling is informally defined as either an explicit call to a
function that scrolls the window, or a pseudo-scroll done by the
display engine when it detects that some part of the window's previous
display is still present, but in a different vertical position.
Moving point to an arbitrary location is neither.