bug#43405: Tool bar item doesn't align to the right edge

[Robert Pluim]

>>>>> On Tue, 22 Sep 2020 17:39:23 +0300, Eli Zaretskii <eliz@gnu.org> said:

    >> From: Robert Pluim <rpluim@gmail.com>
    >> Cc: 43405@debbugs.gnu.org,  juri@linkov.net
    >> Date: Mon, 21 Sep 2020 22:07:51 +0200
    >> 
    Eli> I'd first try to repeat what we do for :align-to support: insert a
    Eli> stretch glyph of a suitable width, computed using it->last_visible_x
    Eli> and the width of the image for the button that has to be
    Eli> right-justified.  See produce_stretch_glyph (except that most of it is
    Eli> not relevant, since :align-to supports a lot of functionalities).
    >> 
    >> OK, that would work, but then you could only right justify a single
    >> item, which would be different from what you can do with the GTK tool
    >> bar

    Eli> No, it will work with more than one as well, you just need to loop
    Eli> twice over all the buttons and keep track of all those which should be
    Eli> right-justified.

OK, weʼre saying the same thing with different words: everything to
the right of this stretch glyph needs its width calculated (and its
x-coordinate adjusted)

    >> (and I see no reason to restrict this to tool bar buttons, I see
    >> at least org-mode wants to right-justify headline tags)

    Eli> What are "headline tags" in Org, and how are they related?

Org headlines look like this:

* This is a headline                       :tag1:tag2:tag3

The tags are right justified by default, but this is done by inserting
spaces, which fails with non-monospace fonts. Thereʼs a patch to
org-mode to do this by instead inserting a space with an :align-to
property, but that code has to calculate the value to specify in lisp,
which I strongly suspect will turn out to be slow, hence doing it in
redisplay would be better (and perhaps more likely to be accurate).

    Eli> Your original description said:

    >> So let's assume we do this by exending the display spec to allow
    >> 
    >> '(:right-justify t)
    >> 
    >> which would mean to move everything on this line as far to the right
    >> in the window as possible.

    Eli> "Everything on this line" on which line?

Sorry, everything on this line after whatever element has that display
spec. Everything before stays where it is (and only the first
:right-justify property found in the line is acted upon).

    Eli> Do you mean you want to display an entire screen line justified to the
    Eli> right?  Then we already do something like that with R2L lines
    Eli> (although there we also reorder display elements, something you don't
    Eli> need here).

No, I just want to be able to produce:

This is text                          This is right justified

And have the "This is right justified" stay right justified as the
window size and line content changes (unless the user deletes that
stretch glyph in the middle).

If the line became longer than could be displayed in the available
window width, then I think the stretch glyph in the middle would just
be treated as a single space (and hence line continuation/truncation
would work as normal).

Robert