> Maybe something like "lsof | egrep '.ttf|.otf'" would enable you to spot > usage in the picker of a font style that is bold by default, and changing > your configured fonts or uninstalling that font would solve this. > > What fonts are configured in KDE's font settings?
No, I checked this the first day I saw it. The only font these programs load is SegoeUI, the only one I have selected in KDE. As experiment, I configured KDE to use bold SegoeUI, and it displays perfect everywhere, including the places where it is rendered wrong now. I think those text widgets has a particular setting that makes the font rendering broken under Qt 5.9.1, but this is only my guess.
