After updating to 18.04, I can't do anything in the folders list with the mouse. Can't switch to another folder nor expand/contract any of them. I can only navigate using the "Go to" menu and related shortcuts (J, N, Alt++, Alt+-, etc...). Will there be a release where kdepim doesn't break anything?.
After some testing, I think Kontact is at fault. If I start standalone KMail, without Kontact, it works properly. I ended up uninstalling Kontact, because it also made Akregator unusable. At least in my case, Kontact is completely broken in 18.04.
Resolved as per comment in 379522, user no longer uses KDE and is the only person to report this bug.