Re: A solution to display completion candidates after point in a minibuffer

[Gregory Heytings via "Emacs development discussions." <emacs-devel@gnu.org>]

>
> I'd be uneasy using such code without some vague understanding about 
> *why* it works.
>

I studied this in a bit more detail.

What happens in the normal case (with icomplete-mode, icomplete-separator 
set to "\n", and window-scroll-functions set to nil) is the following:

1. redisplay_internal() calls resize_mini_window() and 
redisplay_windows(),

2. redisplay_windows() calls redisplay_window_0(), which calls 
redisplay_window(), and the "recenter" part of redisplay_window() is 
executed.

This happens twice (for each window in the frame) before 
redisplay_internal() returns.  It happens twice because FRAME_GARBAGED_P, 
because the windows have been resized, which means that the "goto retry" 
jump is executed.  This means that run_window_scroll_functions() is called 
twice (which has of course no effect if window-scroll-functions is nil).

When window-scroll-functions is set to 
start-display-at-beginning-of-minibuffer, the same happens, except of 
course that start-display-at-beginning-of-minibuffer is executed twice. 
The first time the "recenter" part of redisplay_window() is executed, 
start-display-at-beginning-of-minibuffer sets w->force_start and w->start. 
Then resize_mini_window() sets w->start again, but redisplay_window() is 
called again, and this time it is its "force_start" part that is executed, 
which calls run_window_scroll_functions(), after which try_window() 
updates w->start to the window-start value set in window-scroll-functions.