Today we came across a need that Glyphs couldn’t meet, I think. Our Thai extension was drawn with positive sidebearings on the marks, to make editing easier (thank you to Rainer for the mark preview plugin, everything is now a lot better than my old way). When exporting UFOs and OTFs, the marks are zero-widthed by collapsing the left sidebearing onto the right sidebearing (at least, that’s what it looks like when opening an exported OTF). This is generally fine.
In our situation, however, the rest of the font (Latin etc) in the master files have the marks drawn on zero width, with their _anchors on the x=0, and the client requested the Thai be done the same way. We patched the UFOs to collapse the sidebearings onto the _anchor, rather than collapsing the lsb onto the right one. Unfortunately, although the actual position of the mark on a base is determined by the anchor and not the sidebearings, a rather convoluted investigation showed the mismatch between the OTF’s feature code and the UFO’s patched outlines resulted in every mark getting positioned in the wrong place. @Mark is now spending quite a lot of time to readjust the UFO outlines to be in the same position as the mark feature expects.
It would be very beneficial to have options for different zero-widthing strategies, either to collapse the lsb onto the right, or to collapse both onto the anchor, or maybe to also centre the mark on x=0. I know the automatic behaviour is usually helpful, but not being able to adjust it is not helpful.
I appreciate that these kinds of requests may not be applicable for everyone, but we’re having increasing problems collaborating with clients who need to incorporate our work in larger projects, who are working in different ways with different editors. Figuring out what might go wrong and troubleshooting is taking quite a bit of time, even when working with extremely knowledgeable professionals. Allowing access to settings like this would ease the burden considerably.