Sooo, when developing fonts with FontForge, I got used to desigining my combining marks on a width of an average letter. The main reason was that it's impossible to select for edition a zero-width glyph, so I'd need to open it in new tab every time I needed to adjust something (prove me wrong?). I never experienced any ill effects of this approach, though I wondered if it would break in some obscure software I never used.
But when I moved my project into FontLab VI (btw just exporting and importing UFO is just a brief prelude to porting the whole project including features... thanks FL), I discovered that the exported font does require combining marks to be zero-width, otherwise there's a gap after the positioned mark.
Any way to collapse width automatically on export in FontLab VI? Or an idea on how to do it manually on a batch of glyphs?
What's interesting to me is where the difference actually comes from. Does FontForge add extra code to GPOS upon export to collapse the advance widths when glyphs are positioned?
Btw my preference (if wrong) is for combining marks to be non-zero-width. This way I see it clearly when positioning is missing or fails and I can do something about it. When it's zero-width (and especially positioned to the left of the sidebearings), it may well look as if positioned, which is a nice fallback in some cases, but produces an ugly illegible clash in others, and inhibits completing the feature properly.