diff --git a/gfx/layers/apz/src/InputBlockState.cpp b/gfx/layers/apz/src/InputBlockState.cpp
index e8587c103c82..26fb6f859d76 100644
--- a/gfx/layers/apz/src/InputBlockState.cpp
+++ b/gfx/layers/apz/src/InputBlockState.cpp
@@ -13,6 +13,7 @@
 #include "mozilla/Telemetry.h"              // for Telemetry
 #include "mozilla/layers/APZCTreeManager.h" // for AllowedTouchBehavior
 #include "OverscrollHandoffState.h"
+#include "QueuedInput.h"
 
 #define TBS_LOG(...)
 // #define TBS_LOG(...) printf_stderr("TBS: " __VA_ARGS__)
@@ -307,18 +308,28 @@ DragBlockState::HasEvents() const
 }
 
 void
-DragBlockState::DropEvents()
+DragBlockState::DropEvents(nsTArray<UniquePtr<QueuedInput>>* aQueued)
 {
   TBS_LOG("%p dropping %" PRIuSIZE " events\n", this, mEvents.Length());
+  MOZ_ASSERT(aQueued->Length() >= mEvents.Length());
+  for (size_t i = 0; i < mEvents.Length(); i++) {
+    MOZ_ASSERT(aQueued->ElementAt(i)->Block() == this);
+    MOZ_ASSERT(aQueued->ElementAt(i)->Input()->mInputType == mEvents[i].mInputType);
+  }
+  aQueued->RemoveElementsAt(0, mEvents.Length());
   mEvents.Clear();
 }
 
 void
-DragBlockState::HandleEvents()
+DragBlockState::HandleEvents(nsTArray<UniquePtr<QueuedInput>>* aQueued)
 {
   while (HasEvents()) {
     TBS_LOG("%p returning first of %" PRIuSIZE " events\n", this, mEvents.Length());
     MouseInput event = mEvents[0];
+    MOZ_ASSERT(aQueued->Length() > 0);
+    MOZ_ASSERT(aQueued->ElementAt(0)->Block() == this);
+    MOZ_ASSERT(aQueued->ElementAt(0)->Input()->mInputType == event.mInputType);
+    aQueued->RemoveElementAt(0);
     mEvents.RemoveElementAt(0);
     DispatchEvent(event);
   }
@@ -446,18 +457,28 @@ WheelBlockState::HasEvents() const
 }
 
 void
-WheelBlockState::DropEvents()
+WheelBlockState::DropEvents(nsTArray<UniquePtr<QueuedInput>>* aQueued)
 {
   TBS_LOG("%p dropping %" PRIuSIZE " events\n", this, mEvents.Length());
+  MOZ_ASSERT(aQueued->Length() >= mEvents.Length());
+  for (size_t i = 0; i < mEvents.Length(); i++) {
+    MOZ_ASSERT(aQueued->ElementAt(i)->Block() == this);
+    MOZ_ASSERT(aQueued->ElementAt(i)->Input()->mInputType == mEvents[i].mInputType);
+  }
+  aQueued->RemoveElementsAt(0, mEvents.Length());
   mEvents.Clear();
 }
 
 void
-WheelBlockState::HandleEvents()
+WheelBlockState::HandleEvents(nsTArray<UniquePtr<QueuedInput>>* aQueued)
 {
   while (HasEvents()) {
     TBS_LOG("%p returning first of %" PRIuSIZE " events\n", this, mEvents.Length());
     ScrollWheelInput event = mEvents[0];
+    MOZ_ASSERT(aQueued->Length() > 0);
+    MOZ_ASSERT(aQueued->ElementAt(0)->Block() == this);
+    MOZ_ASSERT(aQueued->ElementAt(0)->Input()->mInputType == event.mInputType);
+    aQueued->RemoveElementAt(0);
     mEvents.RemoveElementAt(0);
     DispatchEvent(event);
   }
@@ -656,18 +677,28 @@ PanGestureBlockState::HasEvents() const
 }
 
 void
-PanGestureBlockState::DropEvents()
+PanGestureBlockState::DropEvents(nsTArray<UniquePtr<QueuedInput>>* aQueued)
 {
   TBS_LOG("%p dropping %" PRIuSIZE " events\n", this, mEvents.Length());
+  MOZ_ASSERT(aQueued->Length() >= mEvents.Length());
+  for (size_t i = 0; i < mEvents.Length(); i++) {
+    MOZ_ASSERT(aQueued->ElementAt(i)->Block() == this);
+    MOZ_ASSERT(aQueued->ElementAt(i)->Input()->mInputType == mEvents[i].mInputType);
+  }
+  aQueued->RemoveElementsAt(0, mEvents.Length());
   mEvents.Clear();
 }
 
 void
-PanGestureBlockState::HandleEvents()
+PanGestureBlockState::HandleEvents(nsTArray<UniquePtr<QueuedInput>>* aQueued)
 {
   while (HasEvents()) {
     TBS_LOG("%p returning first of %" PRIuSIZE " events\n", this, mEvents.Length());
     PanGestureInput event = mEvents[0];
+    MOZ_ASSERT(aQueued->Length() > 0);
+    MOZ_ASSERT(aQueued->ElementAt(0)->Block() == this);
+    MOZ_ASSERT(aQueued->ElementAt(0)->Input()->mInputType == event.mInputType);
+    aQueued->RemoveElementAt(0);
     mEvents.RemoveElementAt(0);
     DispatchEvent(event);
   }
@@ -857,18 +888,28 @@ TouchBlockState::Type()
 }
 
 void
-TouchBlockState::DropEvents()
+TouchBlockState::DropEvents(nsTArray<UniquePtr<QueuedInput>>* aQueued)
 {
   TBS_LOG("%p dropping %" PRIuSIZE " events\n", this, mEvents.Length());
+  MOZ_ASSERT(aQueued->Length() >= mEvents.Length());
+  for (size_t i = 0; i < mEvents.Length(); i++) {
+    MOZ_ASSERT(aQueued->ElementAt(i)->Block() == this);
+    MOZ_ASSERT(aQueued->ElementAt(i)->Input()->mInputType == mEvents[i].mInputType);
+  }
+  aQueued->RemoveElementsAt(0, mEvents.Length());
   mEvents.Clear();
 }
 
 void
-TouchBlockState::HandleEvents()
+TouchBlockState::HandleEvents(nsTArray<UniquePtr<QueuedInput>>* aQueued)
 {
   while (HasEvents()) {
     TBS_LOG("%p returning first of %" PRIuSIZE " events\n", this, mEvents.Length());
     MultiTouchInput event = mEvents[0];
+    MOZ_ASSERT(aQueued->Length() > 0);
+    MOZ_ASSERT(aQueued->ElementAt(0)->Block() == this);
+    MOZ_ASSERT(aQueued->ElementAt(0)->Input()->mInputType == event.mInputType);
+    aQueued->RemoveElementAt(0);
     mEvents.RemoveElementAt(0);
     DispatchEvent(event);
   }
diff --git a/gfx/layers/apz/src/InputBlockState.h b/gfx/layers/apz/src/InputBlockState.h
index 5862723d4f48..124904cec843 100644
--- a/gfx/layers/apz/src/InputBlockState.h
+++ b/gfx/layers/apz/src/InputBlockState.h
@@ -197,13 +197,13 @@ public:
   /**
    * Throw away all the events in this input block.
    */
-  virtual void DropEvents() = 0;
+  virtual void DropEvents(nsTArray<UniquePtr<QueuedInput>>* aQueued) = 0;
 
   /**
    * Process all events using this input block's target apzc, leaving this
    * block depleted. This input block's apzc must not be nullptr.
    */
-  virtual void HandleEvents() = 0;
+  virtual void HandleEvents(nsTArray<UniquePtr<QueuedInput>>* aQueued) = 0;
 
   /**
    * Return true if this input block must stay active if it would otherwise
@@ -235,8 +235,8 @@ public:
 
   bool SetContentResponse(bool aPreventDefault) override;
   bool HasEvents() const override;
-  void DropEvents() override;
-  void HandleEvents() override;
+  void DropEvents(nsTArray<UniquePtr<QueuedInput>>* aQueued) override;
+  void HandleEvents(nsTArray<UniquePtr<QueuedInput>>* aQueued) override;
   bool MustStayActive() override;
   const char* Type() override;
   bool SetConfirmedTargetApzc(const RefPtr<AsyncPanZoomController>& aTargetApzc,
@@ -320,8 +320,8 @@ public:
                  const MouseInput& aEvent);
 
   bool HasEvents() const override;
-  void DropEvents() override;
-  void HandleEvents() override;
+  void DropEvents(nsTArray<UniquePtr<QueuedInput>>* aQueued) override;
+  void HandleEvents(nsTArray<UniquePtr<QueuedInput>>* aQueued) override;
   bool MustStayActive() override;
   const char* Type() override;
 
@@ -357,8 +357,8 @@ public:
   bool HasReceivedAllContentNotifications() const override;
   bool IsReadyForHandling() const override;
   bool HasEvents() const override;
-  void DropEvents() override;
-  void HandleEvents() override;
+  void DropEvents(nsTArray<UniquePtr<QueuedInput>>* aQueued) override;
+  void HandleEvents(nsTArray<UniquePtr<QueuedInput>>* aQueued) override;
   bool MustStayActive() override;
   const char* Type() override;
   bool SetConfirmedTargetApzc(const RefPtr<AsyncPanZoomController>& aTargetApzc,
@@ -507,8 +507,8 @@ public:
   uint32_t GetActiveTouchCount() const;
 
   bool HasEvents() const override;
-  void DropEvents() override;
-  void HandleEvents() override;
+  void DropEvents(nsTArray<UniquePtr<QueuedInput>>* aQueued) override;
+  void HandleEvents(nsTArray<UniquePtr<QueuedInput>>* aQueued) override;
   void DispatchEvent(const InputData& aEvent) const override;
   bool MustStayActive() override;
   const char* Type() override;
diff --git a/gfx/layers/apz/src/InputQueue.cpp b/gfx/layers/apz/src/InputQueue.cpp
index 616eede0ca5b..2e1a8a23730a 100644
--- a/gfx/layers/apz/src/InputQueue.cpp
+++ b/gfx/layers/apz/src/InputQueue.cpp
@@ -26,6 +26,7 @@ InputQueue::InputQueue()
 
 InputQueue::~InputQueue() {
   mInputBlockQueue.Clear();
+  mQueuedInputs.Clear();
 }
 
 nsEventStatus
@@ -174,6 +175,7 @@ InputQueue::ReceiveTouchInput(const RefPtr<AsyncPanZoomController>& aTarget,
   }
   if (!MaybeHandleCurrentBlock(block, aEvent)) {
     block->AddEvent(aEvent.AsMultiTouchInput());
+    mQueuedInputs.AppendElement(MakeUnique<QueuedInput>(aEvent.AsMultiTouchInput(), *block));
   }
   return result;
 }
@@ -233,6 +235,7 @@ InputQueue::ReceiveMouseInput(const RefPtr<AsyncPanZoomController>& aTarget,
 
   if (!MaybeHandleCurrentBlock(block, aEvent)) {
     block->AddEvent(aEvent.AsMouseInput());
+    mQueuedInputs.AppendElement(MakeUnique<QueuedInput>(aEvent.AsMouseInput(), *block));
   }
 
   if (DragTracker::EndsDrag(aEvent)) {
@@ -295,6 +298,7 @@ InputQueue::ReceiveScrollWheelInput(const RefPtr<AsyncPanZoomController>& aTarge
   // MaybeHandleCurrentBlock() does.
   if (!MaybeHandleCurrentBlock(block, event)) {
     block->AddEvent(event);
+    mQueuedInputs.AppendElement(MakeUnique<QueuedInput>(event, *block));
   }
 
   return nsEventStatus_eConsumeDoDefault;
@@ -379,6 +383,7 @@ InputQueue::ReceivePanGestureInput(const RefPtr<AsyncPanZoomController>& aTarget
   // MaybeHandleCurrentBlock() does.
   if (!MaybeHandleCurrentBlock(block, event)) {
     block->AddEvent(event.AsPanGestureInput());
+    mQueuedInputs.AppendElement(MakeUnique<QueuedInput>(event.AsPanGestureInput(), *block));
   }
 
   return result;
@@ -702,15 +707,15 @@ InputQueue::ProcessInputBlocks() {
     // target may be null here if the initial target was unconfirmed and then
     // we later got a confirmed null target. in that case drop the events.
     if (!target) {
-      curBlock->DropEvents();
+      curBlock->DropEvents(&mQueuedInputs);
     } else if (curBlock->IsDefaultPrevented()) {
-      curBlock->DropEvents();
+      curBlock->DropEvents(&mQueuedInputs);
       if (curBlock->AsTouchBlock()) {
         target->ResetTouchInputState();
       }
     } else {
       UpdateActiveApzc(curBlock->GetTargetApzc());
-      curBlock->HandleEvents();
+      curBlock->HandleEvents(&mQueuedInputs);
     }
     MOZ_ASSERT(!curBlock->HasEvents());
 
@@ -760,6 +765,7 @@ InputQueue::Clear()
   APZThreadUtils::AssertOnControllerThread();
 
   mInputBlockQueue.Clear();
+  mQueuedInputs.Clear();
   mActiveTouchBlock = nullptr;
   mActiveWheelBlock = nullptr;
   mActiveDragBlock = nullptr;
diff --git a/gfx/layers/apz/src/QueuedInput.cpp b/gfx/layers/apz/src/QueuedInput.cpp
index 0f63a82d835c..21dd8e1b4e75 100644
--- a/gfx/layers/apz/src/QueuedInput.cpp
+++ b/gfx/layers/apz/src/QueuedInput.cpp
@@ -38,5 +38,17 @@ QueuedInput::QueuedInput(const PanGestureInput& aInput, PanGestureBlockState& aB
 {
 }
 
+InputData*
+QueuedInput::Input()
+{
+  return mInput.get();
+}
+
+CancelableBlockState*
+QueuedInput::Block()
+{
+  return mBlock.get();
+}
+
 } // namespace layers
 } // namespace mozilla
diff --git a/gfx/layers/apz/src/QueuedInput.h b/gfx/layers/apz/src/QueuedInput.h
index 1c9183f3d1ab..68dfbc3c5acb 100644
--- a/gfx/layers/apz/src/QueuedInput.h
+++ b/gfx/layers/apz/src/QueuedInput.h
@@ -39,6 +39,9 @@ public:
   QueuedInput(const MouseInput& aInput, DragBlockState& aBlock);
   QueuedInput(const PanGestureInput& aInput, PanGestureBlockState& aBlock);
 
+  InputData* Input();
+  CancelableBlockState* Block();
+
 private:
   // A copy of the input event that is provided to the constructor. This must
   // be non-null, and is owned by this QueuedInput instance (hence the