don't stop clock when changing visibility

src/WindowBehavior.ts

@@ -23,7 +23,6 @@ export const saveBeforeExit = (app: Editor): null => {
   app.currentFile().candidate = app.view.state.doc.toString();
   app.currentFile().committed = app.view.state.doc.toString();
   app.settings.saveApplicationToLocalStorage(app.universes, app.settings);
-  app.clock.stop();
   return null;
 };