I had the problem of missing window decorations too and also other functionality of kwin stopped working (right-click menu, Alt+mouse for dragging windows).
I upgraded with aptitude safe-upgrade and that turned out to be not enough. By doing an aptitude full-upgrade, the packages libegl1-nvidia and libnvidia- eglcore got installed and that fixed the problem for me. HTH, Diederik -- GPG: 0x138E41915C7EFED6
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