A follow-up from my previous reply.
A few replies back I saw a reference to the Mint Upgrade Tool. I'm used to using the message that appears in MintUpdate when it is time to Upgrade. Out of curiosity I checked the Repos and sure enough there 'is' a MintUpgrade Tool in there. So I installed it, just to be ready when the time comes.
I noticed it was not installed, so I installed it, and when I opened it, it offered to upgrade my system from Mint 20.3 to Mint 21.
I'm aware that there's been no announcement yet (At least at the point where I installed the Upgrade tool). I just decided to see what all the bugginess referred to by the OP here was about .
So now the MintUpgrade Tool has completed.
Anyway, I followed ALL the steps and I am in Mint 21 now.
There was ONE spot where I made a mistake. I received a message that a whole heap of software that is NOT IN The Repos would be removed and that I should select the stuff I wanted to keep, using Preferences.
I couldn't find 'Preferences' anywhere, so I clicked 'Next' and it deleted a heap of stuff I would normally have had to reinstall anyway (which is what I am about to do). Most other things seem to be working at this stage. Some things are unhappy just now but I suspect I'll get them sorted with a couple of tweaks.
For example:
It seems to have got rid of my Printer Drives in CUPS and left me only with the Driverless ones. That means I now have to install the Brother LPR drivers form their support site to get all my special settings back.
The upside is that the Driverless option seems to work ok on my Brother MFC-J6520 and also picked up the A3 sized scanner via Xsane
It screwed up my Private Internet Access VPN App (Fixed using its Settings and a bit of guesswork)
It got rid of my Cairo-Dock (fixed using Synaptic)
It installed most of Plasma 5, though I'm sure I didn't have quite that much of it. (my fault for using some Plasma/KDE stuff over Xfce for theming)
All in all though it seems to be working nicely so far.
Among the other things that are NOT working properly.
There's some 'lag' when typing things like this post.
The Volume Icon is missing from the Panel (should be easy to fix).
I no longer have Video Wallpaper (should be easy to fix)
I don;t have VNC using NoMachine (Hope that's easy to fix)
Not too buggy at all and I would say so far it is NOT the most buggy release in Mint History. I had some awful problems (probably user issues) with one of the earlier releases, years ago.
The most annoying thing is the slight keyboard lag. Definitely something I didn't have in Mint 20.3 - but not a real drama.
I'm off to start checking other stuff.
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U P D A T E
NoMachine is installed and working - VNC is finding and displaying/controlling the other computers currently on the network.
Video Wallpaper is working, so I have a video of an a live aquarium, or any other video displaying as wallpaper - or an animated gif as wallpaper.
(that's one that is totally unnecessary - but I like having, just because I can)
Cairo-Dock is installed and working again
I think the only 'messy' thing I will have to do is manually instal the Brother printer again just because of my special settings.
All my other printers were picked up automatically and can happily operate 'driverless'.