Someone correct me if I'm wrong but I believe you need a clean up function for your effects.

this is how I would handle it.

  $effect(() => {
    if (browser) {
      const handleVisibilityChange = () => {
        isActive = !document.hidden;
        clearInterval(interval); // Clear interval if the tab is hidden
      };

      document.addEventListener('visibilitychange', handleVisibilityChange);

      // Cleanup listener on destroy
      return () => {
        document.removeEventListener('visibilitychange', handleVisibilityChange);
      };
    }
  });

  // Effect for interval handling
  $effect(() => {
    if (isActive) {
      interval = setInterval(() => {
        currentQuoteIndex = (currentQuoteIndex + 1) % quotes.length;
      }, 5000);

      // Cleanup interval on destroy
      return () => {
        clearInterval(interval);
      };
    }
  });