Ps: if you want to reset the site, just press “F5” to refresh the site, and the Metabar will be there again for you.
When you press the button to "kill" the metabar, you are calling this style (css file), which simply "hides" from your web browser all the metabar that Medium shows each time we are reading content.
If you find an error, or want to help, or know more about this idea, you can go to the Kill-Metabar-Maximized Github repository.