Prevent panel going out of screen boundaries

On multiscreen (seems to happen only on X11 so far) when screen are
added and moved, sometimes the panel gets moved by the window manager
ending up out of screen boundaries, leaving  a screen without  panel
a common scenario is:
laptop with external screen connected, panel on internal screen,
close the lid, everything gets moved to the external screen,
the panel  will stay out of screen boundaries, but the panel struts are correct

CCBUG:438114