So I ran aptitude dist-upgrade on my stretch box this afternoon. It updated libkdecorations2-5 and libkdecorations2private5 both to 4:5.3.2-2+b1. It also upgraded libkf5archive5 to 5.12.0-1+b1, along with 40 other packages which I don't think are related to this problem. Once again, KDE krashes as it did before. However, since breeze is already at the sid version, I can't upgrade it. How do I get my desktop back?
Could the the kf5 libraries be held in sid and sent to testing in such a way that these breakages don't occur? If it weren't for the earlier advice to update breeze, I'd still be out of an operating system. This is no good!
