--- a/gfx/layers/apz/src/AsyncPanZoomController.cpp
+++ b/gfx/layers/apz/src/AsyncPanZoomController.cpp
@@ -990,32 +990,36 @@ nsEventStatus AsyncPanZoomController::Ha
     return nsEventStatus_eIgnore;
   }
 
   if (!GetApzcTreeManager()) {
     return nsEventStatus_eConsumeNoDefault;
   }
 
   if (aEvent.mType == MouseInput::MouseType::MOUSE_UP) {
+    APZC_LOG("%p ending drag\n", this);
     SetState(NOTHING);
     ScrollSnap();
     return nsEventStatus_eConsumeNoDefault;
   }
 
   HitTestingTreeNodeAutoLock node;
   GetApzcTreeManager()->FindScrollThumbNode(aDragMetrics, node);
   if (!node) {
+    APZC_LOG("%p unable to find scrollthumb node with viewid %" PRIu64 "\n", this, aDragMetrics.mViewId);
     return nsEventStatus_eConsumeNoDefault;
   }
 
   if (aEvent.mType == MouseInput::MouseType::MOUSE_DOWN) {
+    APZC_LOG("%p starting scrollbar drag\n", this);
     SetState(SCROLLBAR_DRAG);
   }
 
   if (aEvent.mType != MouseInput::MouseType::MOUSE_MOVE) {
+    APZC_LOG("%p discarding event of type %d\n", this, aEvent.mType);
     return nsEventStatus_eConsumeNoDefault;
   }
 
   const ScrollbarData& scrollbarData = node->GetScrollbarData();
   MOZ_ASSERT(scrollbarData.mScrollbarLayerType == layers::ScrollbarLayerType::Thumb);
   MOZ_ASSERT(scrollbarData.mDirection.isSome());
   ScrollDirection direction = *scrollbarData.mDirection;
 
@@ -1030,32 +1034,34 @@ nsEventStatus AsyncPanZoomController::Ha
     ScrollDirection otherDirection = GetPerpendicularDirection(direction);
     ParentLayerCoord distance = GetAxisStart(otherDirection,
         thumbRect.DistanceTo(aEvent.mLocalOrigin));
     ParentLayerCoord thumbWidth = GetAxisLength(otherDirection, thumbRect);
     // Avoid triggering this condition spuriously when the thumb is
     // offscreen and its visible region is therefore empty.
     if (thumbWidth > 0 && thumbWidth * snapMultiplier < distance) {
       isMouseAwayFromThumb = true;
+      APZC_LOG("%p determined mouse is away from thumb, will snap\n", this);
     }
   }
 
   RecursiveMutexAutoLock lock(mRecursiveMutex);
   CSSCoord thumbPosition;
   if (isMouseAwayFromThumb) {
     thumbPosition = aInitialThumbPos;
   } else {
     thumbPosition = ConvertScrollbarPoint(aEvent.mLocalOrigin, scrollbarData) -
                     aDragMetrics.mScrollbarDragOffset;
   }
 
   CSSCoord maxThumbPos = scrollbarData.mScrollTrackLength;
   maxThumbPos -= scrollbarData.mThumbLength;
 
   float scrollPercent = thumbPosition / maxThumbPos;
+  APZC_LOG("%p scrollbar dragged to %f percent\n", this, scrollPercent);
 
   CSSCoord minScrollPosition =
     GetAxisStart(direction, mFrameMetrics.GetScrollableRect().TopLeft());
   CSSCoord maxScrollPosition =
     GetAxisStart(direction, mFrameMetrics.GetScrollableRect().BottomRight()) -
     GetAxisLength(direction, mFrameMetrics.CalculateCompositionBoundsInCssPixelsOfSurroundingContent());
   CSSCoord scrollPosition = minScrollPosition + (scrollPercent * (maxScrollPosition - minScrollPosition));
 
@@ -1063,16 +1069,17 @@ nsEventStatus AsyncPanZoomController::Ha
   scrollPosition = std::min(scrollPosition, maxScrollPosition);
 
   CSSPoint scrollOffset = mFrameMetrics.GetScrollOffset();
   if (direction == ScrollDirection::eHorizontal) {
     scrollOffset.x = scrollPosition;
   } else {
     scrollOffset.y = scrollPosition;
   }
+  APZC_LOG("%p set scroll offset to %s from scrollbar drag\n", this, Stringify(scrollOffset).c_str());
   mFrameMetrics.SetScrollOffset(scrollOffset);
   ScheduleCompositeAndMaybeRepaint();
   UpdateSharedCompositorFrameMetrics();
 
   return nsEventStatus_eConsumeNoDefault;
 }
 
 nsEventStatus AsyncPanZoomController::HandleInputEvent(const InputData& aEvent,

--- a/gfx/layers/apz/src/InputQueue.cpp
+++ b/gfx/layers/apz/src/InputQueue.cpp
@@ -698,23 +698,25 @@ InputQueue::SetConfirmedTargetApzc(uint6
     NS_WARNING("input block is not a cancelable block");
   }
   if (success) {
     ProcessQueue();
   }
 }
 
 void
-InputQueue::ConfirmDragBlock(uint64_t aInputBlockId, const RefPtr<AsyncPanZoomController>& aTargetApzc,
-                                   const AsyncDragMetrics& aDragMetrics)
+InputQueue::ConfirmDragBlock(uint64_t aInputBlockId,
+                             const RefPtr<AsyncPanZoomController>& aTargetApzc,
+                             const AsyncDragMetrics& aDragMetrics)
 {
   APZThreadUtils::AssertOnControllerThread();
 
-  INPQ_LOG("got a target apzc; block=%" PRIu64 " guid=%s\n",
-    aInputBlockId, aTargetApzc ? Stringify(aTargetApzc->GetGuid()).c_str() : "");
+  INPQ_LOG("got a target apzc; block=%" PRIu64 " guid=%s dragtarget=%" PRIu64 "\n",
+    aInputBlockId, aTargetApzc ? Stringify(aTargetApzc->GetGuid()).c_str() : "",
+    aDragMetrics.mViewId);
   bool success = false;
   InputData* firstInput = nullptr;
   InputBlockState* inputBlock = FindBlockForId(aInputBlockId, &firstInput);
   if (inputBlock && inputBlock->AsDragBlock()) {
     DragBlockState* block = inputBlock->AsDragBlock();
     block->SetDragMetrics(aDragMetrics);
     success = block->SetConfirmedTargetApzc(aTargetApzc,
         InputBlockState::TargetConfirmationState::eConfirmed,