Comment 18 for bug 193108

I think this is happening due to the call to pango_font_find_shaper in gfxPangoFonts.cpp line 501: the implementation for PangoFcFont simply returns the first shaper that claims to cover the relevant script, without considering the engine's covers() function. This causes Mozilla to call the Graphite shaper even for a non-Graphite font, which doesn't work very well.

I'll let someone else figure out whether this should be fixed in Mozilla or Pango....