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Re: display property with newline


From: Stephen Berman
Subject: Re: display property with newline
Date: Thu, 03 Jan 2008 23:16:38 +0100
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/23.0.50 (gnu/linux)

On 04 Jun 2007 I posted the following to emacs-pretest-bugs:

> When a display text property contains a newline and some characters
> following the newline, the latter are not displayed when the display
> property is at (or perhaps when it overlaps) (window-start).  To
> reproduce: 
>
> 1. emacs -Q
>
> 2. Type "test " 17 times, i.e., enough to make the line of text longer
> than fill-column.
>
> 3. Put the cursor on column 70 and type
>    `M-: (put-text-property (1- (point)) (point) 'display " \n     ")'
> (the display string here is one space + a newline + five spaces, but
> it is only important that it contain a newline and a following
> character).
>
> Now the buffer displays the following, where `|' marks the left
> fringe, and the cursor is still on column 70, indicated by `^':
>
> |test test test test test test test test test test test test test test 
> |     test test test 
>       ^
>
> 4. Type M-: (scroll-up 1); now the buffer displays the following,
> where again `|' marks the left fringe, and the cursor is still on
> column 70, indicated by `^':
>
> |test test test
>  ^

On Sun, 30 Dec 2007 14:38:11 -0500 Richard Stallman <address@hidden>
replied to me:

> It seems to me that it is impossible to avoid this behavior,
> because the window-start value says to start the redisplay
> with the t that starts `test'.

Must window-start have such a value, and if so why?  Can you (or someone
else) point me to where in the code the window-start value says to start
the redisplay like this (with the t that starts `test')?  I would very
much like to understand why the desired display is impossible with the
current redisplay code.

> So it's more of a misfeature than a bug.

Do you foresee no reasonable change to the redisplay code that could
correct this "misfeature"?  (Why don't you consider it a bug in the
redisplay of the display property?  It sure looks like one to me.)

Steve Berman





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