Frozen window carcass that does not intend to leave
I had specifically the very same trouble. It is not excellent yet I located removing the lower job bar appeared to allow me remove the 'ghost' of the icy plasma widget also.
If you intend to recover the KDE desktop computer to default, removing the.kde directory in your house directory will certainly do it. After that simply log back in (back up your.kde dir simply in instance, though!).
Remember that you'll shed various other KDE details setups, also (like your keyring setups, and more). There IS a details documents in the.kde dir you can delete to reset plasma/desktop - details things, yet when I attempted it it just appeared to function by getting rid of the entire directory - which is why I advise backing it up first.