Bug 1869928 - Only sync throbber animations for tabs that are animating. r=mconley,tabbrowser-reviewers

Differential Revision: https://phabricator.services.mozilla.com/D196537

browser/base/content/tabbrowser.js

@@ -784,11 +784,12 @@
         const animations = Array.from(
           aTab.container.getElementsByTagName("tab")
         )
+          .filter(tab => tab.hasAttribute("busy"))
           .map(tab => {
             const throbber = tab.throbber;
             return throbber ? throbber.getAnimations({ subtree: true }) : [];
           })
-          .reduce((a, b) => a.concat(b))
+          .reduce((a, b) => a.concat(b), [])
           .filter(
             anim =>
               CSSAnimation.isInstance(anim) &&