So currently i have behavior as described in the topic. It is a bit of annoyance since i have applications menu under left mouse button. I would like to change it, but so far i have did not found option to do so. Can anybody help me please?

It seems desktop clicking triggers "show desktop", what't more a plasma-desktop than a KWin thing.
However, I just checked the default plasma desktop containment settings, and while you can bind the left mouse button, you apparently cannot bind it to desktop showing.

-> What button do you press on precisely what desktop containment?

Any button. Default desktop. But now it get back to normal somehow. It is weird.

I could only assume that "something™" had a passive grab or an eventfilter on the mousebutton, or plasma-desktop falsely did not accept (="suck") the event, ie. a bug in event processing.

Do you use any plasmoid or so that would show the desktop on mouseclicks?

I use only default plasmoids (and no show desktop). The only thing that i did before that is to set mouse to left hand mode, but i can't replicate it by doing it again. So i don't think that it was actual reason. This seems to be so random…