bug#15894: 24.3.50; unwarranted horizontal auto-scrolling

bug#15894: 24.3.50; unwarranted horizontal auto-scrolling

[Dani Moncayo]

From "emacs -Q":
1. M-x toggle-truncate-lines
2. Visit a file with a single long line (say, about 300 characters)
3. Move point to the right until horizontal auto-scrolling take
   place. (note that now the point is centered horizontally in the
   window - ok).
4. Now move the point a bit to the left, but not too much, so that
   horizontal auto-scrolling doesn't happen again, and thereby the point
   ends up in a position closer to the left edge of the window.
5. C-x b C-x b (i.e. display another buffer and then display our buffer
   again).

I observe that the cursor is re-centered horizontally, which IMO is not
TRT.  The cursor should be at the same position it was last time I
displayed the buffer (i.e. where I put it).

In GNU Emacs 24.3.50.1 (i686-pc-mingw32)
 of 2013-11-12 on LEG570
Windowing system distributor `Microsoft Corp.', version 6.1.7601
Configured using:
 `configure --enable-checking 'CFLAGS=-O0 -g3' CPPFLAGS=-DGLYPH_DEBUG=1'

Important settings:
  value of $LANG: ESN
  locale-coding-system: cp1252
  default enable-multibyte-characters: t


-- 
Dani Moncayo

bug#15894: 24.3.50; unwarranted horizontal auto-scrolling

[Eli Zaretskii]

severity 15894 wishlist
thanks

> Date: Thu, 14 Nov 2013 14:59:09 +0100
> From: Dani Moncayo <dmoncayo@gmail.com>
> 
> >From "emacs -Q":
> 1. M-x toggle-truncate-lines
> 2. Visit a file with a single long line (say, about 300 characters)
> 3. Move point to the right until horizontal auto-scrolling take
>    place. (note that now the point is centered horizontally in the
>    window - ok).
> 4. Now move the point a bit to the left, but not too much, so that
>    horizontal auto-scrolling doesn't happen again, and thereby the point
>    ends up in a position closer to the left edge of the window.
> 5. C-x b C-x b (i.e. display another buffer and then display our buffer
>    again).
> 
> I observe that the cursor is re-centered horizontally, which IMO is not
> TRT.  The cursor should be at the same position it was last time I
> displayed the buffer (i.e. where I put it).

This isn't a bug, but a missing feature.  Emacs was never coded to
support what you expect.  The amount of hscroll is recorded in the
window object, so when another buffer is displayed in a window, that
information is thrown away and recomputed from scratch.  IOW, Emacs
does not remember what was the hscroll with which the buffer was last
displayed.

Patches are welcome to add such a feature.