Flume does not offer a full-screen view for various reasons. You may however use the macOS Split-View feature (macOS 10.11 or later) to use Flume and another application side-by-side.
To activate Split View mode:
- Hold the OPTION key while long-pressing/clicking on the main window's zoom button (the green button in the title bar area)
- You will be able to choose on which side of your display you would like to keep Flume.
- Select another application that will be displayed beside Flume in Split-View full-screen mode.