GPOS error on exporting variable font -- workaround

On trying to export a variable font, I’m getting this error:
I found this forum thread and tried the Use Extension Kerning parameter, but it didn’t help. And this really shouldn’t be happening anyway, because the file doesn’t contain a lot of kerning yet!

Am running 3.0.4 (3099), but this happened with an earlier version too. The source is a big *.glyphspackage, which can be accessed here (it’s the italic).

Thanks for the report. I fixed it.

Thanks, Georg! I’ll look forward to the update.

This error is still occurring, though I am running 3.0.4 (3100). Is there something I can do to avoid it?
The source is here.

Bumping this because the error is still occurring.

Consider a Use Extension Kerning parameter in File > Font Info > Font.

I tried that, as I wrote in the first post and as you can see in the current source. In fact, there is almost no kerning in this font as of yet.

This problem, whatever it is, is something weirder.

In case anyone passes this way again, here’s how I worked through this problem. I decided that there was something in my font tripping up Glyphs so that it wouldn’t export a variable font. Perhaps there was a bug in Glyphs, perhaps not. But the problem was only with the italic face, not the accompanying roman, so if there was a bug it was likely to be rare and obscure, affecting very few fonts.

I used the Remove Features parameter to figure out that the problem was with anchors, not kerning, and I spent several hours fruitlessly poking through the italic anchors, trying to figure out what was wrong (it wasn’t that there were too many anchors in the italic, which had fewer than the roman).

Finally I decided to write a script that would go through the italic font, trashing the anchors in each glyph and replacing them with anchors copied from the roman font, but retaining the italic position attribute where possible. (Since the collection of anchors in the roman is more complete than in the italic, this means that I’ll have to fix the positions of a lot of anchors newly added to the italic. But I was going to have to do that sooner or later anyway.)

The upshot is that the font now exports as either static or variable.

The problem is most likely that all composite glyphs will get the anchors from the components. That is often useful to be able to attach another mark on top of the existing mark. You can prevent this by adding a parameter called Propagate Anchors and uncheck it.

This is good to know, and I’ll bank it for future reference. But I like the way anchors propagate in Glyphs, and as it’s all working properly now I’m going to let it be.

FWIW, I’m no longer thinking of this as a bug in Glyphs. But it is an area where a more specific error message would be useful—especially which glyph was being processed when the error occurred. I never managed to track that down.

I could export the font Junicode-width.glyphspackage from the link you provided. But after this statement, I am not sure this is the file you are referring to. It has more than six times a thousand kerning pairs, including many exceptions, different for each master. That means that the interpolations must have more exceptions. Combined with large groups, this can lead to a GPOS overflow. The exports worked fine for me with the parameter, in Glyphs build 3100.

It was the italic, with fewer anchors. But it’s good to know that exceptions can increase the chance of a GPOS overflow. I’ll try to address this.