Comment 4 for bug 1573345

I think I can confirm this.
I did not have this behaviour in 15.04, which I used until recently with the same setup for years.

When I upgraded to xenial, I got the same behaviour, just the other way around: I do have primary screen set to the laptop (LVDS1), while my monitor is secondary (using a larger resolution).
Upon disconnecting HDMI and closing the lid, plasmas main bar gets resized as if its on the external monitor (so it gets moved over and back?), reconnecting hdmi with closed lid however moves primary screen to the monitor - otherwise I would not have even realized, why the main bar gets resized in such a weird fashion.

Logging out and back in fixes the problem.
Graphics card is intel.