Suppress content access to page visibility API
The document.visibility property and the visibilitychange event let content know if the user has selected or deselected a tab. If the user switches from tab A to tab B, then tab A can receive a "hidden" event at the same time that tab B receives a "visible" event. So it seems potentially useful to suppress this information.
See https://developer.mozilla.org/en-US/docs/Web/API/Page_Visibility_API
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