I have this bug much time ago. I don't remember exactly but probably since Plasma 5.3. The problem is that when I'm working in Plasma, regarless of the application(s) open, suddenly mouse pointer can move only along a horizontal line and it autoclicks controls of the foremost application at that moment.
I didn't issue a bug report before because it has an easy workaround: I just type Ctrl-Alt-F2 or any other F-key except F1, to open a console screen, then I do nothing there, justo type Ctrl-Alt-F1 to return no Plasma and the cursor is fixed.

Reproducible: Always

Steps to Reproduce:
1. Start Plasma session.
2. Work in any application.


Actual Results:  
Suddenly the mouse pointer can move just along an horizontal line. Rarely (less than 1% of times) along a vertical line. It autoclicks whatever controls are in its path.

Expected Results:  
Mouse pointer should reach every pixel of the screen.

The workaround is to change to a console screen (Ctrl-Alt-F-key) then return to Plasma (Ctrl-Alt-F1)