This is great!!! For this version of NWjs 0.26.6 (26th February 2018) --in-process-gpu seems to be required to be added in the "chromium-args" properties in package.json which is found in package.nw for the overlay to work properly.
Also it seems black lines that appear from letterbox are pretty bad for the achievement popup. I would use scale inner or something else.
Thank you, Scirra! This plugin will be really useful. :)
Will support of Steam Leaderboards be added?