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posn-at-point and display properties

From: ata
Subject: posn-at-point and display properties
Date: Thu, 24 Feb 2011 01:14:45 -0800 (PST)
User-agent: G2/1.0

I believe this to be a bug, but I'm not certain, so I'm posting here
first.  For reference, I'm using 23.2.1 on MacOS.

When posn-at-point is called with an argument POS that's within a text
interval with a non-nil display property, the column it reports is the
column after whatever is displayed.  But (and this is what's confusing
me) it reports the same column for a POS at the start of such an
interval.  Thus, when the cursor is positioned immediately *before* a
display, posn-at-point will return a column for point that is at the
*end* of the display.

This can lead navigation commands to behave in unintuitive fashion.
For example, it will cause next-line to jump columns when line-move-
visual is non-nil. Evaluation of the following form illustrates the

  (goto-char 1)
  (insert "foo foo foo\n" "bar bar bar\n")
  (put-text-property 5 8 'display "XXX")
  (goto-char 5); point is now before XXX, in column 4
  (setq col1 (current-column))
  (setq line-move-visual t)
  (next-line); point is now on the next line, in column 7
  (setq col2 (current-column)))

 => 4
 => 7

The same problem arises with pos-visible-in-window-p, and seems
ultimately traceable to pos_visible_p from xdisp.c.

Is this behavior intended, or should I submit a bug report?

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