Users are required to enable this feature as it is not available as a default enabled option. When enabled the feature, then users when creating messages will see a new window appearing at the center of the users inbox, which in turn expands across most of the viewing screen, resulting in a an improved user experience.
This latest development follows on the recent brand new compose feature on Gmail from a few months ago, which included new tools and a new design to enable users to compose messages where a new window appears without the user having to leave the inbox section.
The way it works for users that haven't enabled the option, a user can simply switch to full screen by clicking the expand button in the top right window. Below is a screenshot of the new full-screen feature.
Feel Free to add your comments to this post, thank you.