After upgrading F39 (KDE) to 40, my SDDM theme failed to run with a module “QtQuickControls” version 1.1 is not installed error. I thought that might indicate it wasn’t compatible with Plasma 6, so I tried a freshly downloaded one and got “QtGraphicalEffects” is not installed instead (no version was specified the second time). The only mention of qtgraphicaleffects a package search finds is a qt5 package, so maybe I was right, but if so why does it offer themes that aren’t compatible with the current Fedora version? I’m missing something fundamental here.
Edit 1: I googled enough to see that qt5 is installed with Plasma 6, so I went ahead and checked some more. qt5-qtgraphicaleffects is installed. I guess sddm themes can’t use it for some reason.