We need to measure the time to switch panel in the Settings app. For that we'll need to send one event to |window| that the performance test will be able to receive. - first, in the performance test, trigger a panel switch by (for example) triggering a touch event on one item in the first panel. The performance test will save the time. - then the app will send an event when the new panel is displayed and ready to be used. The event name can be "x-moz-perf-panel-switch-ready".