diff --git a/gfx/layers/apz/src/AsyncPanZoomController.cpp b/gfx/layers/apz/src/AsyncPanZoomController.cpp
index 97d12679b0a..51ae4c49be8 100644
--- a/gfx/layers/apz/src/AsyncPanZoomController.cpp
+++ b/gfx/layers/apz/src/AsyncPanZoomController.cpp
@@ -3149,6 +3149,7 @@ AsyncPanZoomController::ReportCheckerboard(const TimeStamp& aSampleTime)
   if (!mCheckerboardEvent && (recordTrace || forTelemetry)) {
     mCheckerboardEvent = MakeUnique<CheckerboardEvent>(recordTrace);
   }
+  mPotentialCheckerboardTracker.InTransform(IsTransformingState(mState));
   if (magnitude) {
     mPotentialCheckerboardTracker.CheckerboardSeen();
   }
@@ -3634,7 +3635,6 @@ void AsyncPanZoomController::DispatchStateChangeNotification(PanZoomState aOldSt
 
   if (RefPtr<GeckoContentController> controller = GetGeckoContentController()) {
     if (!IsTransformingState(aOldState) && IsTransformingState(aNewState)) {
-      mPotentialCheckerboardTracker.TransformStarted();
       controller->NotifyAPZStateChange(
           GetGuid(), APZStateChange::TransformBegin);
 #if defined(XP_WIN) || defined(MOZ_WIDGET_GTK)
@@ -3645,7 +3645,6 @@ void AsyncPanZoomController::DispatchStateChangeNotification(PanZoomState aOldSt
       }
 #endif
     } else if (IsTransformingState(aOldState) && !IsTransformingState(aNewState)) {
-      mPotentialCheckerboardTracker.TransformStopped();
       controller->NotifyAPZStateChange(
           GetGuid(), APZStateChange::TransformEnd);
 #if defined(XP_WIN) || defined(MOZ_WIDGET_GTK)
diff --git a/gfx/layers/apz/src/PotentialCheckerboardDurationTracker.cpp b/gfx/layers/apz/src/PotentialCheckerboardDurationTracker.cpp
index 0f9a1d32d86..c83b9f45c24 100644
--- a/gfx/layers/apz/src/PotentialCheckerboardDurationTracker.cpp
+++ b/gfx/layers/apz/src/PotentialCheckerboardDurationTracker.cpp
@@ -40,21 +40,29 @@ PotentialCheckerboardDurationTracker::CheckerboardDone()
 }
 
 void
-PotentialCheckerboardDurationTracker::TransformStarted()
+PotentialCheckerboardDurationTracker::InTransform(bool aInTransform)
 {
-  MOZ_ASSERT(!mInTransform);
-  if (!Tracking()) {
-    mCurrentPeriodStart = TimeStamp::Now();
+  if (aInTransform == mInTransform) {
+    // no-op
+    return;
   }
-  mInTransform = true;
-}
 
-void
-PotentialCheckerboardDurationTracker::TransformStopped()
-{
-  MOZ_ASSERT(mInTransform);
-  mInTransform = false;
   if (!Tracking()) {
+    // Because !Tracking(), mInTransform must be false, and so aInTransform
+    // must be true (or we would have early-exited this function already).
+    // Therefore, we are starting a potential checkerboard period.
+    mInTransform = aInTransform;
+    mCurrentPeriodStart = TimeStamp::Now();
+    return;
+  }
+
+  mInTransform = aInTransform;
+
+  if (!Tracking()) {
+    // Tracking() must have been true at the start of this function, or we
+    // would have taken the other !Tracking branch above. If it's false now,
+    // it means we just stopped tracking, so we are ending a potential
+    // checkerboard period.
     mozilla::Telemetry::AccumulateTimeDelta(
         mozilla::Telemetry::CHECKERBOARD_POTENTIAL_DURATION,
         mCurrentPeriodStart);
diff --git a/gfx/layers/apz/src/PotentialCheckerboardDurationTracker.h b/gfx/layers/apz/src/PotentialCheckerboardDurationTracker.h
index b6bd3ea882b..6154003ad63 100644
--- a/gfx/layers/apz/src/PotentialCheckerboardDurationTracker.h
+++ b/gfx/layers/apz/src/PotentialCheckerboardDurationTracker.h
@@ -39,15 +39,10 @@ public:
   void CheckerboardDone();
 
   /**
-   * This should be called when a transform is started. Calls to this must be
-   * interleaved with calls to TransformStopped().
+   * This should be called at composition time, to indicate if the APZC is in
+   * a transforming state or not.
    */
-  void TransformStarted();
-  /**
-   * This should be called when a transform is stopped. Calls to this must be
-   * interleaved with calls to TransformStarted().
-   */
-  void TransformStopped();
+  void InTransform(bool aInTransform);
 
 private:
   bool Tracking() const;