I’m developing a plugin to generate a proof for any open font. It has a simple Vanilla UI with a dropdown menu to choose from the list of open fonts. Without a unique ID for each font, my plugin needs to use the index of the user-chosen font in Glyphs.fonts. Those indexes can change, and I have found too many scenarios where my list of open fonts gets out of sync despite tracking DOCUMENTCLOSED, DOCUMENTOPENED, and DOCUMENTACTIVATED. I cannot make my plugin’s font chooser dropdown work based on font family name (might be the same for multiple fonts) or document file path (doesn’t work for new, unsaved documents) or the ID of the first master (could potentially change without notice). What I really need is a permanent ID for the font—just like how Glyphs assigns one to each font master.
I’m sure my needs are one of many possible situations where a permanent font document ID would be useful to a developer. This is pretty fundamental. There is no other 100% reliable way of identifying (and differentiating between) two font objects, without storing entire copies of them.