bug#57424: Perceptibility of display-fill-column-indicator-mode

[Christopher Dimech <dimech@gmx.com>]

Have followed the customisation discussion about
display-fill-column-indicator-mode

Have consulted with Protesilaos Stavrou, author of
modus-themes.  I agree that the default indicator character is
too thin.  Changing the character to something wider does not
change things much.  A solution that works is that of setting a
background colour, rather than changing the foreground colour .

Cannot see any consideration of accessibility in its design, as
it looks that the plan had been to make the indicator faint and
almost imperceptible.  This strategy is the worst possible thing
one can do for the cohort of visually challenged people (remember
that perceptibility changes with age, meaning that developers
could be too young and inexperienced to understand
perceptibility considerations).

Protesilaos described how the default style of
display-fill-column-indicator-mode gets a dashed vertical line.
These dashes appear further apart and feel broken if the user (i)
has a font with a tall line height, (ii) increases the value of
line-spacing, (iii) has an increased height for constructs like
headings in Org.

The only way to ensure a contiguous vertical line is to
explicitly set the :height of the face to a small value.  This
brings out the perceptibility problem.

Studying carefully WCAG 2.1, Section 1.4.3 Contrast (Minimum),
the contrast ratio is also subject to change according to the
thinness of the characters.  For instance, With larger thicker
text, the contrast ratio of at least 4.5:1 can be modified to to
at least 3:1.

I see the requirement of a contiguous indicator line is mainly about
aesthetics.  But practical usability should be of higher concern.