The window
Full screen, always on top, and how that differs on each operating system.
The app does not use native fullscreen, deliberately. On macOS a native-fullscreen app gets its own Space, and a three-finger swipe slides straight past it to the desktop.
Instead the window is frameless, fixed, sized to the whole display, and kept at
screen-saver level so it sits above the Dock and menu bar. On macOS it also
joins every Space, so swiping brings the window along rather than revealing
what is behind it.
A site that asks for fullscreen with its own button still gets it. The bar slides away and Esc brings it back.
