with the following ~/.config/fontconfig/fonts.conf:
Looks properly on Qt 5.8:
But wrong on Qt 5.9:
I was able to bisect this to the following change in qtbase: https://codereview.qt-project.org/#/c/171930/
Replace QDrawHelperGammaTables with QColorProfile
Turns the two set of tables in QDrawHelperGammaTables into two
QColorProfile classes that use similar structures and can be reused for
other gamma correction.
At the same time clean-up and improve the comma-correct blending code
to use the new profiles and QRgba64 precision.