Comment 6 for bug 1596751

Seems reasonable, but note that isn't the current behavior of the "automatic" setting. It positions the panels based on monitor number (according to Gdk), but the monitor numbers change if you rearrange the monitors' logical positions, which is what causes the behavior you see. The output selection code was added specifically to fix this problem, because output names do not change if you rearrange the monitors.

It should be very easy to add a pseudo-output called "Primary" which would lock the panel to the primary monitor, regardless of which one it is.