Oh, my. I mistakenly did the unsupported "apt-get upgrade" - I misread some of the last posts here mentioning "apt-get update" while on my smartphone, tried issuing that command and everything
seemed OK, so I went with it. It occurred to me only after things were installing already that I remember reading something on this forum regarding "apt-get upgrade"...
The reason I didn't simply go with dist-upgrade was that it wanted to remove a lot of stuff like plasma-desktop, kde-workspace, dolphin and a bunch of other kde stuff, along with apps like ardour, audacity, vlc, inkscape etc.
And I think I should be under case 1) as I didn't do a DU after July 29, but I did do a couple of selected manual upgrades like "apt-get install linux-image..." and also some system stuff like mesa libs, even of some kf5 stuff etc.
So I'm not sure if what I'm experiencing is due to any of what
I did (wrong), as I explained above, but here are my experiences trying to DU this system.
After apt-get upgrade, there were still some, albeit less stuff that DU would remove, so I selectively did some installs that didn't seem to remove half of my system at once, only some libs that were replaced.
At the end, I got "stuck" with libtag: I couldn't install libtag1v5 without first manually removing libtag1c2a which took most of the KDE with it (at that point I went with it, because I couldn't start KDE anymore anyway), but I reinstalled most of it back with kde-standard, bringing back the rest manually.
One of the packages I had to reinstall manually was kscreen and that gave me problems until I figured out which package is causing it and removed it. With kscreen installed, I'd get a black screen after logging in (either on dock, connected with an external monitor or off the dock and an external monitor). The only thing I'd see after "systemctl start sddm.service" is the KDE splash screen with the progress bar and after that completed, it all went dark.
Without kscreen I can see my desktop, but now I rely upon xrandr to configure my display and at first I thought there was a bug in KDE causing the panel's settings not to be respected (windows would go under panel, even if that wasn't allowed), trying to offset the display using xrandr made everything "magically" work again, even after reverting the offset, so I'm not sure what all of that was about.
Everything else seems to be in order now.
After going through all of this, the latest DU at the moment wants to remove gtk3-engines-oxygen and I'm not sure I should let that go (I use quite a few gtk apps under KDE and I do want them integrated nicely, I did use the Oxygen theme prior to KF5, I'm on Breeze now):
The following packages will be REMOVED:
gtk3-engines-oxygen
The following NEW packages will be installed:
libgck-1-0 libgcr-3-common libgcr-base-3-1
The following packages have been kept back:
dmeventd dmsetup libdevmapper-event1.02.1 libdevmapper1.02.1 liblvm2app2.2 liblvm2cmd2.02 lvm2
The following packages will be upgraded:
firmware-zd1211 gir1.2-gtk-3.0 gvfs gvfs-backends gvfs-bin gvfs-common gvfs-daemons gvfs-libs isc-dhcp-client isc-dhcp-common libcap2 libcap2-bin libgail-3-0 libgtk-3-0 libgtk-3-bin libgtk-3-common libhttpclient-java
libhttpcore-java libhttpmime-java libjna-java libjna-jni
21 upgraded, 3 newly installed, 1 to remove and 7 not upgraded.
Need to get 27.0 MB of archives.
UPDATE: OK, after checking
https://tracker.debian.org/pkg/oxygen-gtk3 I guess that's gone now (removed from unstable and testing), so I might as well let go it to?