Menus are not vertically constrained to the monitor
Quite easy to reproduce, just organize two monitor, disposing them vertically aligned (can just use nested mode).
Right click to open the bg menu in the bottom side of the first monitor:
I expect this to be the same issue of bug https://bugzilla.gnome.org/show_bug.cgi?id=786886 which from what I see should have been fixed but I expect that the missing
this.sourceActor made it not to work.