In some styles the top or left sections of box-shadows would go missing
even though they had non-zero thickness. This was because the inner
local coordinates could become very slightly negative and were then
incorrectly judged to be invisible.
Negative coordinates should not be ignored because that's where the
entire top/left sections of shadow are meant to exist. It's only the
sections of shadow with zero thickness that we should skip.
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1417>
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