I used to be able to middle click on the desktop to get the switch-window menu, but now I can't get any action to map to that button, even though middle click menus work everywhere else (including the panel)...I can still get right/back/forward/all-the-other mouse buttons to work on the desktop...Is this a known Plasma bug? Is the distro (Manjaro) to blame?

So if you right click on the desktop and click "Configure Desktop and Wallpaper", then "Mouse Actions"... What do you have set there for the Middle-Button?

https://paste.opensuse.org/8848660

Operating System: openSUSE Tumbleweed 20201205
KDE Plasma Version: 5.20.4
KDE Frameworks Version: 5.76.0
Qt Version: 5.15.2
Kernel Version: 5.9.12-1-default
OS Type: 64-bit
Processors: 8 × Intel® Core™ i7-4810MQ CPU @ 2.80GHz
Memory: 31.0 GiB of RAM
Graphics Processor: Mesa DRI Intel® HD Graphics 4600

I am having the same issue (also on Manjaro). It doesn't seem to matter what you set the middle button to (in "Configure Desktop and Wallpaper" > "Mouse Actions") - middle clicking on the desktop only takes focus away from the current window. Middle-click paste stays active regardless of the setting.

What greg said, and if it helps to add this: The case holds for when compositing is on/off