Thanks to the WebContents Occlusion feature in Chromium, the Page Visibility Web API will now reflect accurately the visibility state of web pages, especially when they are occluded. In other words, the document.visibilityState value will be “hidden” when browser tab/window is covered by one or multiple window(s).
Docs: https://docs.google.com/document/d/1tPuJE0Vws7oskjjqrawNfMkwFtGxyHV33yQagiplVfY/edit