I didn't use the desktop for a week, then today booted up and **no kwin**!! Plasma would start ok but no window decorations, no moving or resizing windows, no raising windows, the default konsole started with its top well above the smaller monitor (maybe it was level with the larger). Other things like shifting focus with the mouse were glitchy. Altogether unusable. I tried various stuff to no avail, including booting as another user.
I found that kwin was not running, and couldn't find a way to restart it.
So, I tried reverting to a snapper snapshot from a week ago. I have kubuntu on @r and @h (which the installer calls @ and @home), so using (from the btrfs root)
I picked one from the 18th, and the corresponding command on @h to pick the one with the same date. I snapshotted these to @r-new and @h-new, and linked @_link and @home_link to them, as I've got a grub entry for those. I edited the @_link/etc/fstab to use the links instead of @r and @h. Rebooted, still no kwin. But it worked on that day! 
So I tried a pair from the 17th, still failing. What? I tried a pair from the 11th, still failing. So I went back to the earliest snapper snapshot, from the 1st. Still failing. But... I'd got something wrong with that one, so I tried it again, and *KDE and kwin were good*. By looking at what processes were running, I could see /usr/bin/kwin_x11, one of the commands I'd tried to restart it.
Back to the main install, I find that ***kwin had been uninstalled***.
(note a hyphen rather than an underscore) and it's fixed.
Some update before the 11th must have uninstalled kwin-x11, but a problem didn't surface until after a reboot. (I usually just suspend after using the desktop.)
Now I do look at what apt says it's going to do, and have an aptitude script to list packages with their descriptions. And, thanks to the confusion caused by phased updates, I've been looking at the packages more closely in the last month. But still I missed this.
When I found that kwin was not running, I should have suspected that it was because it was not there. When something is gone, it's hard to find out about it, especially when I had to boot into another install to do searches.
I found that kwin was not running, and couldn't find a way to restart it.
So, I tried reverting to a snapper snapshot from a week ago. I have kubuntu on @r and @h (which the installer calls @ and @home), so using (from the btrfs root)
Code:
sudo grep date @r/.snapshots/*/info.xml
So I tried a pair from the 17th, still failing. What? I tried a pair from the 11th, still failing. So I went back to the earliest snapper snapshot, from the 1st. Still failing. But... I'd got something wrong with that one, so I tried it again, and *KDE and kwin were good*. By looking at what processes were running, I could see /usr/bin/kwin_x11, one of the commands I'd tried to restart it.Back to the main install, I find that ***kwin had been uninstalled***.
Code:
sudo apt install kwin-x11
Some update before the 11th must have uninstalled kwin-x11, but a problem didn't surface until after a reboot. (I usually just suspend after using the desktop.)
Now I do look at what apt says it's going to do, and have an aptitude script to list packages with their descriptions. And, thanks to the confusion caused by phased updates, I've been looking at the packages more closely in the last month. But still I missed this.
When I found that kwin was not running, I should have suspected that it was because it was not there. When something is gone, it's hard to find out about it, especially when I had to boot into another install to do searches.