Keeping column positions after scrolling

[Juri Linkov]

There are two bugs where vertical window scrolling doesn't keep
column positions:

1. When a character is displayed by multi-character display table
entry, then scrolling moves point to the end of the line.
For example, after evaluating

(progn
  (setq standard-display-table (make-display-table))
  (aset standard-display-table ?~ (make-vector 4 ?~))
  (dotimes (i (* (window-height) 4))
    (insert "~~\n"))
  (goto-char (point-min)))

this can be observed after calling `scroll-up' (by `C-v' or `PgDn').

If `scroll-preserve-screen-position' is t, then this bug occurs only
when point was at the beginning of the window before scrolling.

2. In all other normal cases, scrolling always moves point to the
beginning of the line.  Perhaps this is not a bug, but rather a
misfeature.  Anyhow, it's very inconvenient when window scrolling
doesn't keep column positions, especially during scrolling formatted
tabular data.

And also it's strange that vertical scrolling doesn't respect even
the goal column set by `C-x C-n'.

-- 
Juri Linkov
http://www.jurta.org/emacs/