i'm a moron. noted.

i guess i saw something that reminded me of the cocoa/whatever-osx-UI on the the screenshots. but looking closer now, there is nothing other than atom and chrome, both have custom inconsistent UIs that do not hint the OS they are running on. you are right, it can be linux or anything else.

You can use something like Divvy (Paid - on AppStore) or Spectacle (Free - https://www.spectacleapp.com/) to achieve the same effect

I personally use Divvy because Spectacle didn't exist when I first started using mac