We currently initialize the porthole to the screen size instead of
the monitor's work area we actually want. At the moment this doesn't
matter, as the minimap is created during initialization with the rest
of the overview, so we can expect a work area change that updates the
porthole to the correct values.
That won't be true for minimaps we put on secondary monitors, so make
sure we initialize the porthole to the actual values.
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1735>
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