This document lists known documentation needs regarding the behaviour of OpenType based shaping. Some behaviors are different for different shapers, and these differences should also be noted.
When are marks zeroed? Before GPOS or after GPOS runs?
After a mark is attached to a base glyph, what is the effect on the position of the mark of changing the offset or advance of the base glyph?
Ligatures are involved with ligature attachment. They are formed as part of a GSUB ligature lookup.
For a ligature component that is not a mark, will a mark attach to the component as a base, using Mark Attachment?
In a 1 to many lookup how are marks attached?
What kind of lookup and which glyphs become ligature components
Once a glyph is a ligature component, is it possible to turn it back into a normal base/mark?
What happens? What happens if the first glyph is subsequently adjusted.