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

[Eli Zaretskii <eliz@gnu.org>]

> From: Alex <agrambot@gmail.com>
> Cc: 28771@debbugs.gnu.org
> Date: Tue, 10 Oct 2017 11:54:41 -0600
> 
> 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.

There's some kind of miscommunication here.  For starters, what do you
mean by "text area"?  For me, "text area" is the area of the window
inside the fringes, see the definition in "(elisp)Frame Layout".  But
you must mean something else, because with my interpretation,
specifying pixel width of the text area makes no sense: it is
determined by the window dimensions.

>    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.

This is only true for a header-line and mode-line, right?  Because the
rest of the window lines don't change their alignment when the scroll
bar switches sides, as their pixel count starts where the left fringe
ends, whether or not there's a scroll bar to the left of the fringe.

>    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.

Again, there's some misunderstanding here, because this example shows
"Text" in the header-line flushed all the way to the left, with only
one space before "Test", to account for the left fringe (if I turn off
fringe-mode, that space disappears).  That's not what you are
describing, so I guess the example needs some change?

>    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.

And again, this is limited to header-line, right?

> 2. Suppose you want to align a string to the right edge of the window.

What is "the right edge of the window" in this context?

> Can you put overlays in the header-line?

No, header-line (as any other Lisp string) can only have text
properties.

> That's my use-case at the moment.

If you are talking only about header lines, maybe the solution (if we
need one) doesn't have to be such a general one.  Are there similar
problems with other lines in a window?

> Is there currently no display spec that allows for appending/prepending
> (or would it be hard to add)?

Appending/prepending what and to what kind of text/object?  Sorry,
maybe it's too late, but I have trouble following your
descriptions :-(