On Sat, Mar 21, 2020 at 02:36:07PM +0100, Lele Gaifax wrote:
> <tomas@tuxteam.de> writes:
> 
> > On Sat, Mar 21, 2020 at 12:18:28PM +0100, Lele Gaifax wrote:

[...]

> > M-x customize, then search for "fringe".
> 
> Yes, I found that, but I'd really prefer to understand why/what is causing
> different widths of the left/right fringe, given that a) nothing in my
> customization explicitly set neither fringe-mode, nor fringe-style, nor
> left/right-fringe-width. As said, "emacs -Q" draws them with an equal width...

Quoth the text in "customize fringe-mode"

   "Note that the actual width may be rounded up to ensure that the
    sum of the width of the left and right fringes is a multiple of
    the frame’s character width.  However, a fringe width of 0 is
    never rounded"

I take that to mean that the fringe widths may differ by up to one
pixel. Perhaps your frame's character width is odd? (I don't know
at the moment how to get that).

Cheers
-- t