This page appears when you leave.
The web is built to hold your attention. This page is built to let it go. Its display begins after 30 seconds of stillness and stops the moment you move. The display is the page's idle report, addressed to no one. If you want to read it, sit still.
This page is a screensaver. It protects your screen the way screensavers protected cathode-ray tubes: it will not let its image settle on your attention long enough to leave a mark.
Inactivity: 00:00:30 — display at 00:00:30
The page cannot tell when you stop looking. It can only tell when you stop moving, and it treats stillness as absence. This is a reasonable assumption, and the page stands by it.
The display will begin after 30 seconds of stillness. To read it, hold still.
How the display works
The page watches for movement: the mouse, the keyboard, a touch, a scroll. Thirty seconds without any, and the display begins. The first sign of movement, and it stops. The report is built in your browser from the time you spent away, together with the departures this browser has on record. Nothing is sent anywhere. The departures live only in this browser's storage, and clearing the site's data clears them.
An absence you never return from is not recorded. The page does not count it. It understands.
About the original screensavers
The screensaver series After Dark was introduced by Berkeley Systems in 1989 for the Macintosh and in 1991 for Windows. Its best-known module, Flying Toasters, came from engineer Jack Eastman, who saw a toaster in a kitchen during a late-night programming session and imagined it with wings. The module had a slider for the toast's darkness. Flying Toasters Pro added a choice of music: Wagner's Ride of the Valkyries, or an anthem with karaoke lyrics.
The toasters were litigated twice. In 1993 Berkeley Systems sued Delrina over a module in which Opus the penguin shot the toasters down. The court found infringement, and Delrina changed the wings to propellers. In 1994 Jefferson Airplane sued, claiming the toasters copied the winged toasters on the cover of their 1973 album Thirty Seconds Over Winterland. The case was dismissed; the cover had never been registered as a trademark.
After Dark's founders, Joan Blades and Wes Boyd, later founded MoveOn.org. The co-developer of the Fish! module, Ed Fries, became vice president of game publishing at Microsoft.
Screensavers existed because a cathode-ray tube will burn a fixed image permanently into its phosphor. This page's protection is the same idea, applied to attention. It has not caused any burn-in so far, and it intends to keep it that way.
Sources: Wikipedia's article on After Dark (software) and the module history it cites. Test hooks for the curious: ?idle=1 forces the display, ?idle=0 keeps the page awake, ?delay=5 shortens the threshold to five seconds.