Thanks Daniel! You’re the best. I couldn’t find that one searching for some reason.
This didn’t actually straight-out fix my problem (though it did give me hope). I did find a workaround that makes the problem less severe, but I’m pretty sure there’s still a bug. Read on -
What did fix my problem was making sure that the attachment settings were “baseline” rather than “center.” But I think that only fixed it because then the Y-offset could be 0.0, so there was no scaling problem to be had. My guess is that the reason that the fix (re-creating the accidentals in a different order) works is because the default attachment settings assume left-to-right entry, so your X-offset is 0.0 or at most 1.0 for minimal spacing. The original bug was about X-spacing, so making sure the Attachment Settings let you enter a very small X-offset would naturally fix the spacing-scaling error.
As a more dramatic example, I’ve attached an example where I created the accidentals left-to-right. Attachment settings are “from Middle Right,” “to Bottom Left,” and Y-offset is 7.0. The center of the grace note is aligned with the center of the accidental, and you can see that the Y-offset from the middle-right of the main accidental (the quarter-tone sharp) positions the flat in the same place both times. Attaching a screenshot of the Tonality Editor window as well, so you can clearly see that the undecimal 1/4-tone sharp is the “primary” accidental.
Thanks a lot! Just switched from Lilypond after ~9 years of developing all sorts of extensions and custom Scheme scripts, and so far I dig it. Learning the Lua script is next on the list, once this piece is done.
Edit: This doesn’t actually solve everything, in part because the baselines with Bravura’s Helmholtz-Ellis notation aren’t all correct. The baseline on the 7-limit sharp alteration should actually be where the blue arrow is on the attached file.