I hope that some font guru (Werner?) can step in and give some
feedback.
I've seen this a few days ago but no time to reply, sorry.
I ran into an issue with font rendering in Lilypond where text
characters are far too large. This issue also affected several
other users and is described here
<https://bbs.archlinux.org/viewtopic.php?id=211350>. It turns out
that if the dpi setting is manually set in the fontconfig
fonts.conf file, Lilypond will adopt that dpi setting (rather than
the standard setting used in Lilypond, which is 1200) and render
all text too large.
Before I create an issue on the tracker, some developer should say
if LilyPond or LXQT must be fixed. I have no idea about it.
I think it's a lilypond bug, since setting a DPI value is for stuff
that goes to the screen and/or a selected printer, but lilypond should
produce stuff that does not depend on the DPI.
I also have a proposed patch that fixes the problem described,
although I don't know if this is the proper way to fix it: [...]
Me neither, unfortunately, since I have never worked with Pango.
However, your patch looks reasonable.
I also created a minimal Pango example that reproduces the bug here
<http://stackoverflow.com/questions/40325159/why-is-the-font-map-resolution-reset-when-using-fontconfig-2-11-94>.
Have you already reported the problem to the fontconfig people? Maybe
they can give more advice.
Werner