== The drawback of modern X font handling gets mysterious

Back in [[The drawback of modern X font handling
../unix/ModernXFontDrawback]] I covered how modern X font rendering
happens in the client and so can vary from client to client, going
from nice on one client to bad on another. I illustrated this with
_xterm_ on Fedora and Ubuntu displaying the same font, Fedora well
and Ubuntu badly. I now have a good reason to change to using _xterm_
with modern fonts, so I spent part of today poking at this issue;
the results have turned this into a genuine peculiar mystery.

What I have so far:

* the problem does not happen with all programs on Ubuntu. So far
  _xterm_ and GNU Emacs have the bad font rendering, but Firefox,
  gnome-terminal, and TK-based programs such as _exmh_ and [[tkmsg
  ../programming/TKLike]] do not; they render DejaVu Sans Mono just
  like Fedora does.

* the problem only happens with some monospace fonts, not all of
  them. The Ubuntu machine I was testing on has 11 candidate
  fonts listed by '_fc-list :scalable=true:spacing=mono: family_';
  seven of them show the problem but four do not.

  (The good four are TlwgMono, Tlwg Typo, Courier New, and FreeMono.
  Unfortunately my preferred xterm font is DejaVu Sans Mono.)

* the problem is not the Ubuntu version of xterm, the Ubuntu app-defaults
  file for xterm, or even the Ubuntu Freetype library; I have built
  the Fedora xterm and [[my version of Freetype NiceTrueTypeFonts]]
  on Ubuntu and used the Fedora app-defaults, and the bad rendering
  is still there.

* I've directly set several fontconfig font rendering options that
  might be doing this without changing anything; at this point I
  haven't seen any difference with _autohint_, _weight_, _embolden_,
  or _aspect_ (the last was a wild shot). Similarly, Xft X resources
  ([[cf http://freedesktop.org/wiki/ScreenFontSettings]]) do nothing
  that I can see.

  (Forcing _autohint=true_ actually makes the Fedora font rendering
  slightly but visibly darker while leaving the Ubuntu rendering
  unchanged for both the good and bad programs.)

* the problem doesn't happen with xterm on some FreeBSD machines I
  have handy; they render DejaVu Sans Mono the good way.

Clearly something mysterious is happening in the depths of the Ubuntu
version of Xft or something it calls, but only if it's invoked in the
right (or wrong) way. Unfortunately I don't think there's any good way
for non-experts to see what font rendering choices are being made (the
fontconfig library can be coaxed into some debugging output, but it's
pretty much 'exports only' from what I can see), so I have no idea if
I'll be able to figure out a solution that lets me use the font I want.

(Changing to gnome-terminal is not a solution for me.)