Tab Focus od LKNQ

Tab Focus is a browser extension that prevents websites from knowing when you switch away from their tab. When enabled on a tab, it intercepts all tab-switch detection methods used by pages: the Page Visibility API, visibilitychange events, blur and focus events, pagehide, and animation frame throttling. The page always sees itself as visible and focused, no matter what you are actually doing.
You can enable protection per tab with a single click or hotkey, choose between three spoofing modes (full, visibility only, or focus only), and set auto-disable timers. Auto-rules let you protect specific sites automatically every time they load, and tab groups let you enable protection across a set of sites at once. A clean popup gives you a live view of all open tabs, rule management with wildcard pattern matching, import and export, and a stats panel tracking usage over time.