--- a/gfx/layers/apz/src/InputBlockState.cpp
+++ b/gfx/layers/apz/src/InputBlockState.cpp
@@ -34,16 +34,22 @@ InputBlockState::InputBlockState(const R
   MOZ_ASSERT(mTargetApzc);
   mOverscrollHandoffChain = mTargetApzc->BuildOverscrollHandoffChain();
 }
 
 bool
 InputBlockState::SetConfirmedTargetApzc(const RefPtr<AsyncPanZoomController>& aTargetApzc,
                                         bool aFromTimeout)
 {
+  if (mTargetConfirmed == TargetConfirmationState::eTimedOut && !aFromTimeout) {
+    // The main thread finally responded. We had already timed out the
+    // confirmation, but we want to update the state internally so that we
+    // can record the time for telemetry purposes.
+    mTargetConfirmed = TargetConfirmationState::eTimedOutAndMainThreadResponded;
+  }
   if (mTargetConfirmed != TargetConfirmationState::eUnconfirmed) {
     return false;
   }
   mTargetConfirmed = aFromTimeout ? TargetConfirmationState::eTimedOut
                                   : TargetConfirmationState::eConfirmed;
 
   TBS_LOG("%p got confirmed target APZC %p\n", this, mTargetApzc.get());
   if (mTargetApzc == aTargetApzc) {
@@ -87,16 +93,23 @@ InputBlockState::GetBlockId() const
 
 bool
 InputBlockState::IsTargetConfirmed() const
 {
   return mTargetConfirmed != TargetConfirmationState::eUnconfirmed;
 }
 
 bool
+InputBlockState::HasReceivedRealConfirmedTarget() const
+{
+  return mTargetConfirmed == TargetConfirmationState::eConfirmed ||
+         mTargetConfirmed == TargetConfirmationState::eTimedOutAndMainThreadResponded;
+}
+
+bool
 InputBlockState::IsDownchainOf(AsyncPanZoomController* aA, AsyncPanZoomController* aB) const
 {
   if (aA == aB) {
     return true;
   }
 
   bool seenA = false;
   for (size_t i = 0; i < mOverscrollHandoffChain->Length(); ++i) {
@@ -190,17 +203,17 @@ CancelableBlockState::IsDefaultPrevented
 {
   MOZ_ASSERT(mContentResponded || mContentResponseTimerExpired);
   return mPreventDefault;
 }
 
 bool
 CancelableBlockState::HasReceivedAllContentNotifications() const
 {
-  return IsTargetConfirmed() && mContentResponded;
+  return HasReceivedRealConfirmedTarget() && mContentResponded;
 }
 
 bool
 CancelableBlockState::IsReadyForHandling() const
 {
   if (!IsTargetConfirmed()) {
     return false;
   }

--- a/gfx/layers/apz/src/InputBlockState.h
+++ b/gfx/layers/apz/src/InputBlockState.h
@@ -45,16 +45,17 @@ public:
   {}
 
   virtual bool SetConfirmedTargetApzc(const RefPtr<AsyncPanZoomController>& aTargetApzc, bool aFromTimeout);
   const RefPtr<AsyncPanZoomController>& GetTargetApzc() const;
   const RefPtr<const OverscrollHandoffChain>& GetOverscrollHandoffChain() const;
   uint64_t GetBlockId() const;
 
   bool IsTargetConfirmed() const;
+  bool HasReceivedRealConfirmedTarget() const;
 
   void SetScrolledApzc(AsyncPanZoomController* aApzc);
   AsyncPanZoomController* GetScrolledApzc() const;
   bool IsDownchainOfScrolledApzc(AsyncPanZoomController* aApzc) const;
 
 protected:
   virtual void UpdateTargetApzc(const RefPtr<AsyncPanZoomController>& aTargetApzc);
 
@@ -62,16 +63,17 @@ private:
   // Checks whether |aA| is an ancestor of |aB| (or the same as |aB|) in
   // |mOverscrollHandoffChain|.
   bool IsDownchainOf(AsyncPanZoomController* aA, AsyncPanZoomController* aB) const;
 
 private:
   enum class TargetConfirmationState {
     eUnconfirmed,
     eTimedOut,
+    eTimedOutAndMainThreadResponded,
     eConfirmed
   };
 
   RefPtr<AsyncPanZoomController> mTargetApzc;
   TargetConfirmationState mTargetConfirmed;
   const uint64_t mBlockId;
 
   // The APZC that was actually scrolled by events in this input block.