bug#28771: 26.0.60; A couple space display property feature requests

[Alex <agrambot@gmail.com>]

Eli Zaretskii <eliz@gnu.org> writes:

>> From: Alex <agrambot@gmail.com>
>> Date: Mon, 09 Oct 2017 18:20:42 -0600
>>
>> 1. Specifying the pixel width to the left/center/right of the text area.
>>    This is helpful (I believe it's necessary if I want it to be robust)
>>    when one wants to pad either side of a string with the same number of
>>    pixels and the left side uses (:align-to 0).
>> 
>>    I could use (:width (+ left-fringe left-margin)) for the right
>>    padding, but this doesn't take into account line-number display, as
>>    well as the scroll-bar width (if it's on the left).
>> 
>> 2. Specifying the pixel position relative to the left or right edge of
>>    the window. The right edge of the window currently may be the right
>>    edge of the right fringe or the scroll bar, depending on what side it
>>    is on. Also, just in the case that in the future the
>>    fringe/scroll-bar may not be the outermost elements, there should be
>>    a way to explicitly specify one of the edges.
>
> These two requests are no in the original message, and I admit I don't
> really understand the situations they describe.  Can you elaborate
> about them, preferably with an example that shows what is currently
> possible and an explanation why the current capabilities are
> insufficient?  (Yes, I've seen the patches you propose, but I still
> would like to understand the problems better.)

Right, they're additions that I felt should go directly to a bug report.

1. There is currently no robust way to specify a pixel width to the
   center or left/right edge of the text area in the 'space' display spec.

   This is because there's no way to tell what's before the left edge of
   the text area. If there's a left-side scroll-bar, then its width
   should be included; but if it's on the right, its width shouldn't be.

   :align-to currently allows for this calculation, but :width doesn't.
   The diff adds the same calculation to :width.

   As for an example, try this in a graphical emacs -Q:

(setq header-line-format
      (propertize
       (concat (propertize " "
                           'display
                           '(space :align-to 0))
               "Test"
               (propertize " "
                           'display
                           '(space :width (+ left-fringe left-margin))))
       'face 'highlight))

  
   By default, the spaces before and after test are equally sized. Even
   if you disable fringe-mode or enable margins (e.g. linum-mode), the
   spaces are equal. However, try setting the scroll-bar to appear on
   the left. Now, the left space is noticeably bigger than the right.

   Changing the last space's :width spec to be '(+ left-fringe
   left-margin scroll-bar)' will now make the spaces equal with a left
   scroll-bar, but unequal with a right scroll-bar.

   If you apply my diff and change the second space's :width spec to be
   'left', then it is correct in both cases.

   I don't know of a good use-case for :width center or :width right,
   but I figured that for completeness I might as well implement them.

2. Suppose you want to align a string to the right edge of the window. I
   couldn't find a way to do this currently since there's no guarantee
   what elements are on the left/right edges. Try:

(setq header-line-format
      (concat (propertize " "
			  'display
			  '(space :align-to (- (+ scroll-bar
						  scroll-bar)
					       4)))
	      "Test"))

   This works if there's a right scroll-bar (or if there's none, oddly
   enough), but not if there's a left scroll-bar. Replacing `scroll-bar'
   with `right-fringe' doesn't work if there's a right scroll-bar.

   Replacing the :align-to spec with `right-edge' works in all cases.

   A similar argument can be made for aligning from the left edge.


>> 3. Following from "1." and the help-gnu-emacs thread, it would be nice
>>    if there was a display property that allowed you to pad a string on
>>    both sides with a pixel-specified space instead of surrounding it
>>    with 2 pixel-specified spaces.
>
> I replied on help-gnu-emacs.  In a nutshell, the 'space' display spec
> is a replacing spec, so it cannot be easily used on non-space text.
> You could probably have a before-string whose value is a space with
> the 'space' display spec, if that makes any difference.

Can you put overlays in the header-line? That's my use-case at the
moment.

> use the space-width spec, as described on help-gnu-emacs.

I believe in my case (since the text may contain spaces), that's
equivalent to just the :width spec on the first/last spaces.

#3 is just a convenience request -- it makes dealing with pixel-spaces a
bit easier, but as long as #1 is implemented then I don't think it bring
any new functionality.

Is there currently no display spec that allows for appending/prepending
(or would it be hard to add)? If so, then #3 can be ignored.