I want to create a "Chrome app" to open Gmail in a separate windows Chrome instance on my Mac.
The method for doing this is well documented - https://support.google.com/chrome_webstore/answer/3060053?hl=en-GB
In Chrome, you go to the three dots for Settings > More Tools > Create Shortcut and then tick "Open as new window" when you name the shortcut.
However, when I click on the resulting Gmail icon, it opens in Gmail back in the main Chrome browser window, not a separate app window.