Additional Features:

• Fullscreen on play: Works just like maximize on play, but with fullscreen;
• Mosaic: Lets you configure one game view for each display, this helps testing games that support multiple displays;
• Span: Span one fullscreen window across multiple displays;
• Editor Fullscreen: Fullscreen Unity's main window;
• API: Create, toggle and destroy fullscreen views using C# methods, supports any kind of Editor Window;
• Keep the state: Changes made to the window during fullscreen are kept after exiting fullscreen.

Supported Platforms:

• Windows: All features available, out-of-the-box multi-display fullscreen;
• macOS: Not tested on multi-display setups; Has issues with display scaling on retina (high density) displays;
• Linux: Requires wmctrl to be installed and a EWMH/NetWM compatible X Window Manager; Has some features limitations.

Source code included!