bug#8627: 24.0.50: cursor property behaves irregularly in before-strings

[Eli Zaretskii <eliz@gnu.org>]

> Date: Thu, 05 May 2011 20:01:00 -0400 (EDT)
> From: Alp Aker <aker@pitt.edu>
> 
> The documentation for the cursor property (info node "(elisp) Special 
> Properties") states that:
> 
> "Normally, the cursor is displayed at the end of any overlay and text 
> property strings present at the current buffer position.  You can place 
> the cursor on any desired character of these strings by giving that 
> character a non-`nil' `cursor' text property."
> 
> And then follows the description of using integer values for the cursor 
> property to make it applicable to a range of buffer positions other than 
> the range between the overlay's start and end.

Note that, crucially, the before-string and after-string properties
are not described in this section.  They are described elsewhere,
where the `cursor' property is not mentioned.  So I wonder if the
`cursor' property was really supposed to work with before-strings and
after-strings...

> However, when used with overlay before-strings (or after-strings), the 
> cursor property appears to behave in ways that aren't consonant with the 
> docs and that don't follow a consistent pattern.

It looks like it was never supported consistently, ever since the
`cursor' property was introduced (in Emacs 22.1).

> (1) Insert some boilerplate text to interact with.
> 
>    (save-excursion
>      (goto-char (point-min))
>      (insert "Lorem ipsum dolor sit amet.\n"))
> 
> Make an overlay and give it a before-string with a non-nil cursor property 
> on one character:
> 
>    (setq olay (make-overlay 5 6)
>          str1 (concat "XX"
>                       (propertize "Y" 'cursor t)
>                       "ZZ"))
>    (overlay-put olay 'before-string str1)
> 
> If one now does:
> 
>    (goto-char (overlay-start olay))
> 
> the cursor property is ignored; the cursor appears after the 
> before-string.

This works as expected in Emacs 23, so it must be some problem with
the bidi-aware redisplay in Emacs 24.  I will take a look when I have
time; thanks for a clear test case.

> (3) But if the before-string contains a newline, the cursor property 
> appears to be ignored regardless of location and regardless of whether the 
> value of the cursor property is numeric or merely non-nil.

This never worked, I tested it in Emacs 22.1, and it behaves like this
as well.

I will take a look at fixing this, bat can you show a real-life use
case where this is needed?

> I started to read through xdisp.c to make some sense of this, but I'm too 
> new to Emacs's internals to be able to follow much of the display routine. 
> I did observe, though, that cursor_row_p only checks display properties, 
> and doesn't appear to check before-strings and after-strings.

It may come as a surprise, but the implementation of display
properties is very different from that of before-string and
after-string properties.  It's a small wonder they don't behave
similarly.

> In any case, it would appear that either the treatment of before-strings 
> and after-strings should be changed so that the cursor property works with 
> them as it does with strings that come from display properties, or the 
> documentation should be amended to make clear that that it can't be 
> expected to do so.  (In the latter case, it would still seem desirable 
> that cursor properties in before- and after-strings behave systematically, 
> which isn't currently the case.)

It sounds like it works in most use cases, so fixing the
implementation is probably a better alternative.