i'll post 3 possible candidate patches for this and let the reviewer pick their favorite.

Created attachment 237566 [details] [review]
screen: fix meta_screen_get_monitor_for_rect for 0x0 rects

meta_screen_get_monitor_for_rect will return the monitor that
a given rect belongs in (choosing the "best" monitor based on
overlap, if there are overlapping monitors).

It doesn't work with 0x0 rects, though.

This commit fixes that.

Created attachment 237567 [details] [review]
layout: fix findIndexForActor for point-sized actors

The background menu code uses a 0x0 dummy actor for positioning
a context menu where the user clicks when the user right clicks
on a background actor.

As part of this positioning process, the popup menu code identifies
the monitor the actor should pop up on using LayoutManager's
findIndexForActor function.

This function creates a MetaRectangle from the actor's position
and dimensions and passes the rect to mutter's
get_monitor_index_for_rect() function.  That function doesn't
allow 0x0 rects, though, so the background menu shows up in the
wrong place.

This commit forces the rect passed to mutter to always be atleast 1x1.

Created attachment 237568 [details] [review]
backgroundMenu: force dummy cursor actor to have a size

The background menu code uses a 0x0 dummy actor for positioning
a context menu where the user clicks when the user right clicks
on a background actor.

The popup menu doesn't allow 0x0 sized actors, though,
which causes the menu to get mispositioned.

This commit makes the dummy actor be 1x1 instead of 0x0 to satisfy
the popup menu code's requirements.

Review of attachment 237566 [details] [review]:

I like this one the most.

Attachment 237566 [details] pushed as de36d51 - screen: fix meta_screen_get_monitor_for_rect for 0x0 rects