I fixed it! I had to do
sudo apt install kwin-x11and restart the laptop. Then I get close / maximise buttons, can switch with Alt+Tab, etc.
This was based on this Q&A https://unix.stackexchange.com/questions/261122/kde-5-does-not-have-window-decorations
This seems like a bug in Triskel - surely this package should be installed by default, no?
FYI - the other package suggested in the linked answer (kwin-decoration-oxygen) makes no difference as far as I can tell (I tried it separately).
