Exact same problem here. I was able to recover normal behavior by only downgrading libkdecoration2-5v5:amd64 to version 4:5.17.5-2, so this is definitively a libkdecoration2 issue.
For inexperienced user (like myself) struggling to solve this issue, you can download the file here http://ftp.debian.org/debian/pool/main/k/kdecoration/libkdecorations2-5v5_5.17.5-2_amd64.deb and install it with sudo apt install ./libkdecorations2-5v5_5.17.5-2_amd64.deb Best Victor
