bug#43272: 27.1; variable-pitch-mode misaligns org-mode heading tags

bug#43272: 27.1; variable-pitch-mode misaligns org-mode heading tags

[Protesilaos Stavrou]

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1. Run 'emacs -Q'
2. Open a new Org file 'C-x C-f /tmp/test.org'
3. Insert the following contents:

* TODO this is a test                                                   :tag:
* TODO here is another test                                             :tag:

The tags should align to the right with the standard monospaced font.

4. Run 'M-x variable-pitch-mode'.
5. The tags are now misaligned.

Please see attached screenshots for the before and after states of the
'variable-pitch-mode' activation.

Running 'M-x describe-char' in the space between the heading's title and
the tag informs us that the space's face is the same as the heading's
(e.g. 'org-level-1').

I am also relaying the technical insight of Adam Spiers.  Please see
attached plain text document or visit the comment's URL:
https://gitlab.com/protesilaos/modus-themes/-/issues/85#note_407147422

Overview of my setup:

+ GNU Emacs 27.1 (build 2, x86_64-pc-linux-gnu, GTK+ Version 3.24.22,
  cairo version 1.17.3) of 2020-08-14

+ Org mode version 9.3 (release_9.3 @ /usr/share/emacs/27.1/lisp/org/)

--
Protesilaos Stavrou
protesilaos.com


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I've investigated this and it's not nearly as easy as I had hoped.  It
turns out that the right alignment supported by display properties can
only align space, not text.  Something like this works as an experiment:

```lisp
(with-current-buffer "org/TODO.org"
  (put-text-property (region-beginning) (region-end)
                     'display '(space :align-to (- right 10))))
```

If a region of space characters is selected, the above will adjust the
region's width so that its right hand side is 10 character widths in
from the right margin.  However if the region contains text, it will
make that text invisible.

This could be used to *left*-align the tags, but not *right*-align them,
which doesn't really work because each heading's tags can be a different
number of characters wide.

So to get our desired behaviour of adjusting the space in between the
heading and the tags correctly in order to right-align the tags to the
window's right margin, we'd first have to calculate the display width of
the (proportionally spaced) tags text, and then subtract that from the
right margin.

In theory this width can be retrieved via 'pos-visible-in-window-p' with
the 'PARTIALLY' argument set to 't', but I haven't got that working yet,
and even if I could, it would need to be done dynamically for each line
and then the corresponding text property updated whenever that width
changed (e.g. any of that line was edited, or when the window was
scrolled to bring different headings into view).

However that raises questions about how well this would perform, since
it would be creating quite a lot of work to do on each redisplay.  I
suspect that the emacs gurus would say that support for right-alignment
of variable pitch text would need to be added to emacs's internal
display routines (the ones written in C, not Elisp).  But that's only a
vague guess.  Hopefully I've underestimated what emacs can currently do
here and there's an easier solution, but there's little chance of
finding out either way without asking on the Org list, and then falling
back to the 'help-gnu-emacs' or 'emacs-devel' list if it's even too
difficult a question for any of the Org gurus.

bug#43272: 27.1; variable-pitch-mode misaligns org-mode heading tags

[Bastien]

Hi Protesilaos,

thanks for the report.

I suggest you post the same report to emacs-orgmode@gnu.org, where
more Org contributors may comment and try to help.

I can reproduce this problem and I don't know how to solve it.

As a comment to Adam's explorations: Org tags are not aligned with
respect to the window margin, but wrt to `org-tags-column'.  And I
don't think we can implement a solution without changing this, so
that's quite a change.

But perhaps someone will want to tackle this.

Thanks,

-- 
 Bastien

bug#43272: 27.1; variable-pitch-mode misaligns org-mode heading tags

[Eli Zaretskii]

> From: Bastien <bzg@gnu.org>
> Date: Tue, 08 Sep 2020 14:10:54 +0200
> Cc: 43272@debbugs.gnu.org
> 
> I suggest you post the same report to emacs-orgmode@gnu.org, where
> more Org contributors may comment and try to help.

Yes, please.

> As a comment to Adam's explorations: Org tags are not aligned with
> respect to the window margin, but wrt to `org-tags-column'.  And I
> don't think we can implement a solution without changing this, so
> that's quite a change.

Maybe I'm missing something, but :align-to accepts values in pixels,
so in general you could put a display property on the separating
whitespace and request that the whitespace ends at a certain pixel
coordinate from the left; that coordinate could somehow be computed
from org-tags-column, for example by multiplying it by
frame-char-width.

bug#43272: 27.1; variable-pitch-mode misaligns org-mode heading tags

[Bastien]

Eli Zaretskii <eliz@gnu.org> writes:

> Maybe I'm missing something, but :align-to accepts values in pixels,
> so in general you could put a display property on the separating
> whitespace and request that the whitespace ends at a certain pixel
> coordinate from the left; that coordinate could somehow be computed
> from org-tags-column, for example by multiplying it by
> frame-char-width.

Thanks for the direction, I will try this.

I'm closing this report, as this is not an Emacs bug and it will be
discussed in Org's mailing list.

-- 
 Bastien

bug#43272: 27.1; variable-pitch-mode misaligns org-mode heading tags

[Eli Zaretskii]

> From: Protesilaos Stavrou <info@protesilaos.com>
> Date: Tue, 08 Sep 2020 13:13:04 +0300
> 
> 1. Run 'emacs -Q'
> 2. Open a new Org file 'C-x C-f /tmp/test.org'
> 3. Insert the following contents:
> 
> * TODO this is a test                                                   :tag:
> * TODO here is another test                                             :tag:
> 
> The tags should align to the right with the standard monospaced font.
> 
> 4. Run 'M-x variable-pitch-mode'.
> 5. The tags are now misaligned.
> 
> Please see attached screenshots for the before and after states of the
> 'variable-pitch-mode' activation.

This is the expected behavior: with variable-pitch fonts, characters
no longer have the same width, so aligning text by counting columns no
longer works.  You need pixel-level alignment.

This should be fixed in Org, I think.