When using a second screen, this sseems to be realised as a second QFrame. This ist most times not viewable, because it has no borders, but sometimes the second screen has thes borders. It can be seen in the screenshot (see URL),, where the whole Frame is shown. It happens approximatly very 10th start in Plasma 5.1, stopped a while, but now in Plasma 5.2.2 it appears again. Restarting Kwin soes not solve the problem, only a logout and complete restart helps. Reproducible: Sometimes Steps to Reproduce: 1. start Plasma...
Forgot a detail: The second screen "stands out" over the first screen twice the width of the border (cannot be seen in the screenshot clearly...)
Can't understand what you mean, can't really do much with the current description. It would need a precise description of what happens, maybe with screenshots or even video of the problem.
I think the bug was "my Plasma opens in a window on the second screen". Please try upgrading to Plasma 5.5 and Qt 5.5 and reopen this bug if you can still see this bug.