Freezing a stylistic set can cause other stylistic sets that replace overlapping glyphs to malfunction about fonttools-opentype-feature-freezer

[twardoch]

If you have "a" and lookup1 associated with ss01 replaces "a" with "a.ss01", and lookup2 associated with ss02 replaces "a" with "a.ss02", then when you enable both features and you type U+0061, you get "a" which then gets replaced with "a.ss01" and the lookup associated with ss02 has nothing to replace.

If you "freeze" ss02, then U+0061 gets reassigned from "a" to "a.ss02" so it's the direct input. The lookup associated with ss01 then has no effect because it still expects "a" as input.

The tool is simple — it only reassigns the Unicode to another glyph (the one that is the result of a 1:1 replacement). It doesn't change the way other features work. That would be arcane and unpredictable (esp. in contextual subs). The tool is deliberately simple and works as designed.

[twardoch]

PS. The tool does not remove any glyphs, because a glyph from which it "steals" the Unicode may (or may not) be also used as an output glyph in some feature. So it's safer to "orphan" the glyph. (Because the tool is deliberately simple and does not try to be overly clever).

But you may use pyftsubset to remove these orphaned glyphs afterwards at your discretion.

[twardoch]

In the end, it really depends how the features are written in the font, on the order of the original lookups. The tool only changes the entry points to the OT features maze (reassigns the Unicode codepoints), but it keeps the maze untouched.