Because we're using a simple (gint) cast to round y value of
the clicked point, a negative y was rounding to a valid cell
position and showing the popover even when the user clicked
on the blank cells before the actual month view.
Fix that by properly checking if y is negative.
https://bugzilla.gnome.org/show_bug.cgi?id=755910
Georges Basile Stavracas Neto
b6bec55185