[HarfBuzz] is opentype crazy

[Martin Hosken]

Dear All,

It seems that if I want to insert a glyph (say a dotted circle) between two diacritics, then I can. I can

sub diac1' diac2 by diac1 dottedcircle

or something akin to that. But when I do, because diac1 is a mark and not a ligature, harfbuzz resets the advance of my dottedcircle to 0, even though dottedcircle is listed in gdef as being a base. Is this a bug, he asks hopefully?

GB,
Martin