When I first undocked the windowing system became so confused I needed to force a logout and re-login to be able to work: the windows displayed but I couldn't type into them.

Second time I have adjusted my displays before undocking so that the laptop screen overlapped the monitor. This time undocking worked except the panel went away. When I have conjured it back it became transparent and I don't like that. A reboot have moved the monitor and LCD apart again and the panel is not transparent.

So: how can I make Panel non-transparent? How can I get back to the old KWin behavior where unplugging and replugging a monitor caused an effortless resize? I would like either the monitor or the LCD to display a max res picture but never both.

Transparent panel might be (related to) https://bugs.kde.org/show_bug.cgi?id=337355 (the problem in your case could be that the panel forgets to update the contrast effect region)

KWin doesn't manage the screen layout, but there're plenty of reports on this forum that (dynamic) screen management is broken in various aspects (notably forgetting settings on re-login) - so this is probably just a bug. (Afaiu, setting the screens "unified" *should* do what you want, but likely simply doesn't)

Ah so Kwin is still in the picture despite using SDDM? I wonder whether the good ole' kwin --replace helps anything.

What's the Plasma equivalent of

kquitapp plasma-desktop;sleep 1; plasma-desktop;disown;exit

? (Really what a horrible name choice, it's impossible to Google any KDE5 information!)

X11: display server
KDM/SDDM: display manager
KWin: window manager & compositor (indirect display server in case of wayland)
plasma-desktop/plasmashell: desktop and panel windows
kscreen: output management (it's a kded module)

KWin is "in the picture" because it paints windows (as long as the compositor is active, what's an X11 requirement for transparent windows, inc. panels) - if panels are "too" transparent, this smells like caused by that weird contrast effect approach.

SDDM allows you to select a session, but has no (direct) relation to the session as well, including what happens with the screen layout *during* the session. (Screen issues that only occur for the login screen are a different matter)

you want to run "kquitapp plasmashell; plasmashell &; disown; exit"

To gather all running processes, run "ps aux". Oh how to use google: "plasma AND KDE AND KF5"