All present-day font editors lack the option to define contextual kerning using a UI. I made a bunch of scriptlets that together form a tool to implement contextual kerning for Glyphs 3, hijacking the normal kerning user interface after a contextual kerning "mode" was activated.
It saves the contextual kerning values in Glyphs’ "Numbers" (named data points that can have different values per master) that can be referenced in feature code as variables ("Tokens"). This also works in glyphsLib.
The tool is usable, but I wish that font editors would one day implement such functionality natively, so I labelled it "Proof of Concept". If you run into any trouble using this, please let me know.https://github.com/yanone/Yanone-GlyphsApp-Scripts/tree/main/Contextual Kerning