I recently upgraded to ubuntu 18.04 and noticed a bad behavior of gnome's activities overview:
I have two monitors and work with the option "Workspaces span displays" enabled. In the gnome configuration of ubuntu, however, when I open the activities overview I see all opened windows in every workspace in the secondary display.
Let's say I have firefox open on the secondary display in workspace 1. If I open activities overview in workspace 3 I see this firefox window, although I've nothing open in WS 3.
I've been using ubuntu gnome since its first release and never had such problem.
Is there an additional option to toggle, or shall I submit a bug report?
EDIT: A bug report actually exists on launchpad