Since Xnview MP version 0.84, the UI fonts are badly rendered on my Ubuntu 16.04 system (see first attached screenshot).
After some investigation, I found that the issue comes from my /etc/fonts/local.conf file, that contains the lines:
<match target="font">
<edit name="antialias" mode="assign"><bool>true</bool></edit>
<edit name="hinting" mode="assign"><bool>true</bool></edit>
<edit name="hintstyle" mode="assign"><const>hintfull</const></edit>
<edit name="lcdfilter" mode="assign"><const>lcdlegacy</const></edit>
<edit name="rgba" mode="assign"><const>vrgb</const></edit>
</match>
These options usually give a crisp look with Qt applications. But the line:
<edit name="rgba" mode="assign"><const>vrgb</const></edit>
is responsible for the ugly font rendering in Xnview MP >= 0.84.
If I change this line to:
<edit name="rgba" mode="assign"><const>rgb</const></edit>
then the fonts are well displayed (see second attached screenshot), but of course they are less crispy.
The bug occurs with rgba = vrgb or vbgr, but not with rgba = rgb or bgr.
The bug does not occur with other Qt 5 applications (which are dynamically linked with Qt 5.5.1 on Ubuntu 16.04).
I guess it's a problem with the Qt 5 version that is used to statically link the Xnview MP application.
The bug is 100% reproducible.