Problems with font metrics in improper gravities
When the gravity of a font is improper, we give the hb_font_t a negative scale to flip the advance width. But this has the side effect of also giving us negative font metrics, in particular a negative font height.
This leads to problems with code like https://gitlab.gnome.org/GNOME/pango/-/blob/main/pango/pango-layout.c#L5647 which assumes that font heights are positive.
The visible symptom of this is wrapped lines in those gravities getting rendered all over each other when you set line-spacing to something other than 0.