Fonts are rendered quite differently in QtCreator than with most other applications on my system (see "Environment"). I suspect that it is an issue with xcb vs. X11 libs.

I was able to find this bug which is similar to this issue:
"Font rendering regression on mac"
https://bugreports.qt-project.org/browse/QTBUG-28653

From the attached screenshots of the same text and font in both QtCreator and gedit, you can see that the rendering of even the same two characters next to each other is slightly different in QtCreator than in gedit, which renders them the same (e.g., "tt" and "nn" in "Mittwoch" and "Donnerstag" respectively).

It would be nice if one could choose to use either xcb or X11 at runtime, or else give the option at compile time if building it oneself.