bug#17974: 24.3.92; scrolled wrapped tab starts with wrong position

[YAMAMOTO Mitsuharu]

I'm not sure if this should be counted as a bug.  This is not a
serious one, but causes a strange look in pixel-based smooth scrolling
a window with scaled text in the Mac port.  The recipe below can also
be reproduced in various X11 builds.  Probably this is related to
Bug#17969.

Steps to reproduce:

1. $ emacs -Q -D &
2. (setq frame-resize-pixelwise t) C-j
3. C-x 5 2
4. (progn
     (text-scale-set 5)
     (dotimes (i 10) (insert (make-string 7 (+ ?a i)) ?\t))
     (insert ?\n)) C-j
5. Resize the newly created frame so it looks like the first
   screenshot.  Notice that there is some space before "fffffff"
6. Type "(scroll-up 1)" and repeatedly hit "C-x C-e" until the line
   starting with "fffffff" comes at the top.

Result:

The line starting with "fffffff" is shown without leading space (the
second screenshot) whereas there was some space before the last scroll
up.

                                     YAMAMOTO Mitsuharu
                                mituharu@math.s.chiba-u.ac.jp

In GNU Emacs 24.3.92.1 (x86_64-apple-darwin13.2.0, GTK+ Version 3.12.2)
 of 2014-07-09 on YAMAMOTO-no-iMac.local
Repository revision: 117365 
monnier@iro.umontreal.ca-20140708183807-3hfyv6otyq5tgxcj
Windowing system distributor `The X.Org Foundation', version 11.0.11406000
Configured using:
 `configure --without-imagemagick LDFLAGS=-L/opt/local/lib
 CPPFLAGS=-I/opt/local/include'