--- a/layout/generic/nsGfxScrollFrame.cpp
+++ b/layout/generic/nsGfxScrollFrame.cpp
@@ -1975,19 +1975,20 @@ ScrollFrameHelper::IsSmoothScrollingEnab
   return Preferences::GetBool(SMOOTH_SCROLL_PREF_NAME, false);
 }
 
 class ScrollFrameActivityTracker final : public nsExpirationTracker<ScrollFrameHelper,4> {
 public:
   // Wait for 3-4s between scrolls before we remove our layers.
   // That's 4 generations of 1s each.
   enum { TIMEOUT_MS = 1000 };
-  ScrollFrameActivityTracker()
+  explicit ScrollFrameActivityTracker(nsIEventTarget* aEventTarget)
     : nsExpirationTracker<ScrollFrameHelper,4>(TIMEOUT_MS,
-                                               "ScrollFrameActivityTracker")
+                                               "ScrollFrameActivityTracker",
+                                               aEventTarget)
   {}
   ~ScrollFrameActivityTracker() {
     AgeAllGenerations();
   }
 
   virtual void NotifyExpired(ScrollFrameHelper *aObject) {
     RemoveObject(aObject);
     aObject->MarkNotRecentlyScrolled();
@@ -2534,17 +2535,18 @@ void ScrollFrameHelper::MarkRecentlyScro
   if (IsAlwaysActive()) {
     return;
   }
 
   if (mActivityExpirationState.IsTracked()) {
     gScrollFrameActivityTracker->MarkUsed(this);
   } else {
     if (!gScrollFrameActivityTracker) {
-      gScrollFrameActivityTracker = new ScrollFrameActivityTracker();
+      gScrollFrameActivityTracker = new ScrollFrameActivityTracker(
+        SystemGroup::EventTargetFor(TaskCategory::Other));
     }
     gScrollFrameActivityTracker->AddObject(this);
   }
 
   // If we just scrolled and there's a displayport expiry timer in place,
   // reset the timer.
   ResetDisplayPortExpiryTimer();
 }