Fix disappearing reader mode icon on WNP 129 (Fixes #14975) (#14976)

media/js/firefox/whatsnew/whatsnew-129-na.js

@@ -52,6 +52,18 @@ function init() {
     });
 
     Mozilla.UITour.forceShowReaderIcon();
+
+    // show reader mode icon again on visibility change
+    // see https://github.com/mozilla/bedrock/issues/14975
+    document.addEventListener(
+        'visibilitychange',
+        () => {
+            if (!document.hidden) {
+                Mozilla.UITour.forceShowReaderIcon();
+            }
+        },
+        false
+    );
 }
 
 if (