bug#73129: 31.0.50; buffer-text-pixel-size vs. newlines

[Eli Zaretskii <eliz@gnu.org>]

> Date: Mon, 9 Sep 2024 00:22:11 +0200
> From:  David Ponce via "Bug reports for GNU Emacs,
>  the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>
> 
> Its seems that the function `buffer-text-pixel-size' is not working
> as described when there are newlines in the buffer.  Here is a quick
> illustration (on my laptop (frame-char-width) returns 12 pixels):
> 
> ;; As expected.
> (with-temp-buffer
>    (insert "abcdef")
>    (car (buffer-text-pixel-size nil nil t)))
> 72
> 
> ;; I suppose it is as expected because newline is not displayed.
> (with-temp-buffer
>    (insert "\n")
>    (car (buffer-text-pixel-size nil nil t)))
> 0
> 
> ;; ?
> (with-temp-buffer
>    (insert "abcd\nef")
>    (car (buffer-text-pixel-size nil nil t)))
> 48

The above is the intended behavior, assuming that the default width of
the frame's characters is 12 in your case.  The maximum width of the
text "abcd\nef" is 4 characters ("abcd"), which are 12*4 = 48 pixels.
The part after the newline is only 2 characters, so it is narrower,
and does not affect the result.

IOW, buffer-text-pixel-size measures the 2D _dimensions_ of the text,
not is total width.

> The doc string of `buffer-text-pixel-size' mentions:
> 
> "Return size of whole text of BUFFER-OR-NAME in WINDOW."

It also says:

  The return value is a cons of the maximum pixel-width of any text
  line and the pixel-height of all the text lines of the buffer
  specified by BUFFER-OR-NAME.

Does the above explain the behavior you see?

This is not a bug.

> So ,I expect that the last example will return 72px (6 x 12px) + 0px
> for the newline.  But the result is 48px, which means that all the
> text after the first newline is not counted.

Your expectations are incorrect, see above.

> This also impacts "string-pixel-width" which is supposed to return
> the pixel size of the passed string, not part of it:
> 
> (string-pixel-width "abcdef") => 72
> (string-pixel-width "\n") => 0
> (string-pixel-width "abcd\nef") => 48
> (string-pixel-width "abcd\nef\ngh") => 48

Yes.  If you want the _total_ width of a string, remove the newlines
from it.

I think I understand the source of your confusion, so I have now
clarified these subtle points in the doc strings of
window-text-pixel-size, buffer-text-pixel-size, and
string-pixel-width.

> At least this limitation should be mentioned in the doc string?

I've now done so (and they are not limitations, but conscious design
decisions which have good reasons).

> A possible fix for `string-pixel-width' could be to remove all
> the newlines from the passed string before to call
> `buffer-text-pixel-size'?  Something like this:

Thanks, but that would be the wrong thing.  If the caller wants a
total width, the caller should remover the newlines or measure each
substring separately and add the results.

May I ask where and for what purpose did you need to measure pixel
width of a string that included newlines?