Symptom
There were 5 pageLoaded Popups events logged Sunday, 4th June 2017. This is drastically low.
Cause
As the Page Previews (PP) application boots it [loads the ext.eventLogging.Schema module (asynchronous)](https://github.com/wikimedia/mediawiki-extensions-Popups/blob/master/src/index.js#L146-L150) and then [dispatches the BOOT action (synchronous)](https://github.com/wikimedia/mediawiki-extensions-Popups/blob/master/src/index.js#L152). As part of reducing the action, we create and queue a pageLoaded event for logging. The event is only logged if the eventLogging change listener is registered. Unfortunately, the change listener is registered when the module has loaded, which, if the module has been loaded from the ResourceLoader's cache, will be in the next tick of the JavaScript event loop.
This is an instance of the late subscriber problem.
Possible fix
As I suggested in T158999#3087132 and T158999#3110621, rely on [the event. mw.track protocol defined by the EventLogging extension](https://github.com/wikimedia/mediawiki-extensions-EventLogging/blob/master/modules/ext.eventLogging.subscriber.js) over the mw.eventLog.Schema class/API.