Access the Steamworks API from the NW.Js exporter. NOTE: Where possible consider using the Steamworks for WebView2 plugin instead.
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.)