Google recently made an update to Gmail, which allows users to compose new mail in full-screen mode. When full-screen mode is activated, the compose window doesn't expand to encompass the entire screen, but moves to the center of the screen, while everything beside the compose window is dimmed.
This feature replaces the "pop-out" option that was previously between the "minimize" and "close" buttons on the top right corner of the compose window. This however, doesn't mean the "pop-out" option has been removed. To get the compose windows to pop-out, simply hold the "Shift" button and the pop-out option will reappear.
Users are also able to set the new full-screen compose mode as default by clicking on the menu button at the bottom right corner of the compose window (right beside the "discard" button).
According to a Gmail Google+, this full-screen compose option is a much-requested feature, and will be rolling out to all Gmail users over the next couple of days.
Source: Gmail Google+