bug#18504: 24.3.93; posn-at-point intermittently returns wrong value for a valid buffer position

[Dmitry <dgutov@yandex.ru>]

(Not a regression, same in Emacs 24.3).

I've been trying to understand what triggers this and create a smaller
repro case, but so far had no luck. So we'll use the company package.

1. git clone https://github.com/company-mode/company-mode.git

2. cd company-mode; git checkout foofoofoo

(Note that the last commit in this branch adds a `message' call that
outputs a multiline string. This doesn't seem to be mandatory, but makes
reproduction easier).

3. emacs -L . -Q

4. (require 'company)
M-x global-company-mode
;; Pick another binding if you're in tty
(global-set-key (kbd "C-/") 'company-complete-common)

5. Type `C-o' several times, so that there are a few empty lines under
the current one.

6. Type `  a'. Again not mandatory, but I've had better odds of
reproduction with this line content.

7. Quickly and repeatedly mash `C-/' and `C-g', a lot.

You'll see the popup appearing under the current line and column, but
sometimes noticeably displaced to the right (like by 15 columns). If it
doesn't happen, try mashing the keys in different rhythms, faster or
slower, and kind of simultaneously (not alternating).

After you've seen it, eval (setq bidi-display-reordering nil).

Mash the keys again, now the popup will occasionally be displayed below
its intended position, by 11 lines or so.

If you add some debugging output to `company--col-row', you'll see that,
for the same value of POS, (posn-at-point pos) returns in these odd
moments "position objects" with very different X and Y coordinates.

This bug is most likely related to the `company-pseudo-tooltip-overlay'
now being positioned starting with the end of the current line (before
the newline), because if I add a space after the current point, I can't
reproduce it anymore. This was at one point suggested by Eli
(http://lists.gnu.org/archive/html/emacs-devel/2014-08/msg00328.html),
or at least hinted at.

I'd be happy to help debugging this further, if someone tells me how.

In GNU Emacs 24.3.93.3 (x86_64-unknown-linux-gnu, GTK+ Version 3.10.8)
 of 2014-09-05 on axl
Repository revision: 117482 monnier@iro.umontreal.ca-20140904161426-2072ebabqpyhaadw
Windowing system distributor `The X.Org Foundation', version 11.0.11501000
System Description:	Ubuntu 14.04.1 LTS