Just guessing (I'm not near a Linux system right now): QQC2TextField only exists since QtQuickControls 2.5, but /usr/share/kwincompositing/qml/EffectView.qml imports 2.0: import QtQuick.Controls 2.0 as QQC2 I.e. this bug is probably introduced by upstream commit https://cgit.kde.org/kwin.git/commit/kcmkwin/kwincompositing?h=Plasma/5.12&id=22d898399b38cadfd72474a2b50f1d41db7bd28e