--- a/gfx/layers/apz/src/APZCTreeManager.cpp
+++ b/gfx/layers/apz/src/APZCTreeManager.cpp
@@ -1659,17 +1659,17 @@ APZCTreeManager::GetTargetAPZC(const Scr
   RefPtr<HitTestingTreeNode> node = GetTargetNode(aGuid, nullptr);
   MOZ_ASSERT(!node || node->GetApzc()); // any node returned must have an APZC
   RefPtr<AsyncPanZoomController> apzc = node ? node->GetApzc() : nullptr;
   return apzc.forget();
 }
 
 already_AddRefed<HitTestingTreeNode>
 APZCTreeManager::GetTargetNode(const ScrollableLayerGuid& aGuid,
-                               GuidComparator aComparator)
+                               GuidComparator aComparator) const
 {
   mTreeLock.AssertCurrentThreadOwns();
   RefPtr<HitTestingTreeNode> target = DepthFirstSearchPostOrder<ReverseIterator>(mRootNode.get(),
       [&aGuid, &aComparator](HitTestingTreeNode* node)
       {
         bool matches = false;
         if (node->GetApzc()) {
           if (aComparator) {

--- a/gfx/layers/apz/src/APZCTreeManager.h
+++ b/gfx/layers/apz/src/APZCTreeManager.h
@@ -460,17 +460,17 @@ private:
                                 uint64_t aOriginatingLayersId,
                                 uint32_t aPaintSequenceNumber);
 
   void AttachNodeToTree(HitTestingTreeNode* aNode,
                         HitTestingTreeNode* aParent,
                         HitTestingTreeNode* aNextSibling);
   already_AddRefed<AsyncPanZoomController> GetTargetAPZC(const ScrollableLayerGuid& aGuid);
   already_AddRefed<HitTestingTreeNode> GetTargetNode(const ScrollableLayerGuid& aGuid,
-                                                     GuidComparator aComparator);
+                                                     GuidComparator aComparator) const;
   HitTestingTreeNode* FindTargetNode(HitTestingTreeNode* aNode,
                                      const ScrollableLayerGuid& aGuid,
                                      GuidComparator aComparator);
   AsyncPanZoomController* GetTargetApzcForNode(HitTestingTreeNode* aNode);
   AsyncPanZoomController* GetAPZCAtPoint(HitTestingTreeNode* aNode,
                                          const ParentLayerPoint& aHitTestPoint,
                                          HitTestResult* aOutHitResult,
                                          HitTestingTreeNode** aOutScrollbarNode);