When do you see this? It is either a plugin or something from the macOS text input system.
it pops up when I press
n for a couple of seconds
Then it is the textinput. They the same in TextEdit or Mail …
Some built-in keyboards support this since macOS 10.12, I believe.
Yes, it is a Mac thing, not Glyphs. I can trigger it anywhere, for example browser URL. On my keyboard (international English, setup with US English) when I hold down the /n key for a second, I get this contextual popup.