Access the Steamworks API from the NW.Js exporter. NOTE: Where possible consider using the Steamworks for WebView2 plugin instead.
My example of installing necessary files from Steam SDK and steam_appid.txt to Mac application on MacOS
Not too surprising that overlay did not work on MacOS. Issues around MacOS Steam overlap support being discussed on Greenworks Github issues list: github.com/greenheartgames/greenworks/issues/200 is one example. There are some chromium switches to try out (as JeHawk mentioned --in-process-gpu seems to help, at least for Windows OS Overlay. I will do some experimenting on MacOS. Also, at least on windows, for Overlay to work, game must be launched from Steam Launcher (register as a non-steam game.)