OpenType positioning bug?

I tried to follow this tutorial, Positioning with number values | Glyphs to use positioning method instead of substitution for .case glyphs.
When I write pos o <10 40 20 0>; in my “case” feature and compile nothing happen. Feature button in Preview Panel still gray, and no OpenType Feature are present in exported font.

AFAIK, GPOS is not working in Glyphs edit/preview view.
You need to check GPOS codes with “Text Preview” tool.

Ok, but I can’t enable opentype feature in Text Preview.

You can use the kern table with “# Automatic Code” to test:

@mekkablue It is possible to do it like in the tutorial ?