It's a very convenient feature to change my wallpaper via drag & drop of pictures from Dolphin to the desktop. But if the widget are locked, this dialog doesn't come up after dropping my picture. The mouse pointer shows just a tiny stop sign. This doesn't make much sense to me because on the other hand you can still change your wallpaper with the main wallpaper settings. Reproducible: Always
That doesn't make sense. Confirmed. Thanks.
This is reduced in severity in Plasma 5.18 now that Locked mode is a mostly-hidden sysadmin-only thing, so most regular users will never run into this case. Still, probably worth fixing.
You can now! I think this has probably been fixed since Plasma 5.18, but for sure it's working for me with 5.20.