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pageBeforeRemove and Cleanup

pageBeforeRemove is the last moment the page element exists. Undo there whatever pageInit set up that outlives it: window and document listeners, timers, observers, and widgets from app.*.create(), such as the Actions sheet, whose element lives in the app root:

The cleanup handler in src/pages/book.f7
    $on('pageBeforeRemove', () => {
      sheet.destroy();
      window.removeEventListener('online', onNetwork);
      window.removeEventListener('offline', onNetwork);
    });

With the handler deleted, books 1, 2 and 3 were opened with Back in between, then Playwright 26,280 went offline with context.set_offline(True):

Output of 29
--- go offline
     5 ms  book 1 handles "offline"
     5 ms  book 2 handles "offline"
     5 ms  book 3 handles "offline"

Two removed pages still ran handlers and stayed in memory; with the cleanup, only book 3 printed. Components can also use $onBeforeUnmount.