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Seems that the latest qt update to 5.15.1 breaks lots of stuff. It broke external Qt themes on my Arch installation, too, or, at least the qt5ct + adwaita-qt(AUR) combination. Downgrading everything qt-related back to 5.15.0 fixed the issue.
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AUR packages are unsupported. It's your job to update or rebuild them as necessary.
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Qt5-styleplugins has to be rebuild in order to make qt apps work properly.
Rules for problems.
Everyone has problems. Animals have problems. And buildings. And cats, and trees.
Problems are your friends. Treat them well.
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