Can't add new widgets anywhere by dragging and dropping them, be it the desktop or a panel, have to double click on them. Reproducible: Always Steps to Reproduce: 1.Browse for a widget. 2.Select one 3.Try to drag it (left click+hold) to your desire location. Some widgets will by default be added to the desktop or to the panel, it depends on how you browsed for them. If right click on desktop + Add widget, it will be added to the desktop. If browsed from "a" pane options it will be added to said panel. In either case drag and drop does not work.
Thanks for your bugreport. This was fixed by [1] today. [1] https://quickgit.kde.org/?p=plasma-framework.git&a=commit&h=1ace6a46dedfb8536890f9c86e7b4fbbc55931ac