I have a laptop and use attached monitor as primary display. When panel on the primary display is oriented vertically, dragging window between monitors makes the window disappear slightly after entering secondary display. I've been testing this behaviour in different distros on Xorg with intel video since Plasma 5.8. Iit still reproduces in 5.9. Latest Neon packages (LTS or user edition) can be used to easily reproduce this bug. Video showing this bug is here: https://yadi.sk/i/NeByvNm038yuxm
*** This bug has been marked as a duplicate of bug 371199 ***