diff --git a/gfx/layers/LayerMetricsWrapper.h b/gfx/layers/LayerMetricsWrapper.h
index ab37e281aa35..d78b1e04c578 100644
--- a/gfx/layers/LayerMetricsWrapper.h
+++ b/gfx/layers/LayerMetricsWrapper.h
@@ -406,6 +406,12 @@ class MOZ_STACK_CLASS LayerMetricsWrapper final {
     return mLayer->GetFixedPositionScrollContainerId();
   }
 
+  SideBits GetFixedPositionSides() const {
+    MOZ_ASSERT(IsValid());
+
+    return mLayer->GetFixedPositionSides();
+  }
+
   Maybe<uint64_t> GetZoomAnimationId() const {
     MOZ_ASSERT(IsValid());
     // This function is only really needed for template-compatibility with
diff --git a/gfx/layers/apz/src/APZCTreeManager.cpp b/gfx/layers/apz/src/APZCTreeManager.cpp
index 3033d516863b..da2c58def670 100644
--- a/gfx/layers/apz/src/APZCTreeManager.cpp
+++ b/gfx/layers/apz/src/APZCTreeManager.cpp
@@ -992,7 +992,8 @@ HitTestingTreeNode* APZCTreeManager::PrepareNodeForLayer(
                        : Nothing());
     node->SetScrollbarData(aLayer.GetScrollbarAnimationId(),
                            aLayer.GetScrollbarData());
-    node->SetFixedPosData(aLayer.GetFixedPositionScrollContainerId());
+    node->SetFixedPosData(aLayer.GetFixedPositionScrollContainerId(),
+                          aLayer.GetFixedPositionSides());
     return node;
   }
 
@@ -1216,7 +1217,8 @@ HitTestingTreeNode* APZCTreeManager::PrepareNodeForLayer(
   // when those properties change.
   node->SetScrollbarData(aLayer.GetScrollbarAnimationId(),
                          aLayer.GetScrollbarData());
-  node->SetFixedPosData(aLayer.GetFixedPositionScrollContainerId());
+  node->SetFixedPosData(aLayer.GetFixedPositionScrollContainerId(),
+                        aLayer.GetFixedPositionSides());
   return node;
 }
 
diff --git a/gfx/layers/apz/src/HitTestingTreeNode.cpp b/gfx/layers/apz/src/HitTestingTreeNode.cpp
index 202ad948b1fe..249653ce2028 100644
--- a/gfx/layers/apz/src/HitTestingTreeNode.cpp
+++ b/gfx/layers/apz/src/HitTestingTreeNode.cpp
@@ -133,14 +133,17 @@ const ScrollbarData& HitTestingTreeNode::GetScrollbarData() const {
 }
 
 void HitTestingTreeNode::SetFixedPosData(
-    ScrollableLayerGuid::ViewID aFixedPosTarget) {
+    ScrollableLayerGuid::ViewID aFixedPosTarget, SideBits aFixedPosSides) {
   mFixedPosTarget = aFixedPosTarget;
+  mFixedPosSides = aFixedPosSides;
 }
 
 ScrollableLayerGuid::ViewID HitTestingTreeNode::GetFixedPosTarget() const {
   return mFixedPosTarget;
 }
 
+SideBits HitTestingTreeNode::GetFixedPosSides() const { return mFixedPosSides; }
+
 void HitTestingTreeNode::SetPrevSibling(HitTestingTreeNode* aSibling) {
   mPrevSibling = aSibling;
   if (aSibling) {
diff --git a/gfx/layers/apz/src/HitTestingTreeNode.h b/gfx/layers/apz/src/HitTestingTreeNode.h
index f229aa24871e..a1a252552b67 100644
--- a/gfx/layers/apz/src/HitTestingTreeNode.h
+++ b/gfx/layers/apz/src/HitTestingTreeNode.h
@@ -125,8 +125,10 @@ class HitTestingTreeNode {
 
   /* Fixed pos info */
 
-  void SetFixedPosData(ScrollableLayerGuid::ViewID aFixedPosTarget);
+  void SetFixedPosData(ScrollableLayerGuid::ViewID aFixedPosTarget,
+                       SideBits aFixedPosSides);
   ScrollableLayerGuid::ViewID GetFixedPosTarget() const;
+  SideBits GetFixedPosSides() const;
 
   /* Convert |aPoint| into the LayerPixel space for the layer corresponding to
    * this node. |aTransform| is the complete (content + async) transform for
@@ -183,6 +185,7 @@ class HitTestingTreeNode {
   ScrollbarData mScrollbarData;
 
   ScrollableLayerGuid::ViewID mFixedPosTarget;
+  SideBits mFixedPosSides;
 
   /* Let {L,M} be the {layer, scrollable metrics} pair that this node
    * corresponds to in the layer tree. mEventRegions contains the event regions
diff --git a/gfx/layers/wr/WebRenderScrollDataWrapper.h b/gfx/layers/wr/WebRenderScrollDataWrapper.h
index dcbc673dca6d..962877951f8f 100644
--- a/gfx/layers/wr/WebRenderScrollDataWrapper.h
+++ b/gfx/layers/wr/WebRenderScrollDataWrapper.h
@@ -364,6 +364,13 @@ class MOZ_STACK_CLASS WebRenderScrollDataWrapper final {
     return mLayer->GetFixedPositionScrollContainerId();
   }
 
+  SideBits GetFixedPositionSides() const {
+    MOZ_ASSERT(IsValid());
+
+    // TODO: Implement for WebRender.
+    return eSideBitsNone;
+  }
+
   Maybe<uint64_t> GetZoomAnimationId() const {
     MOZ_ASSERT(IsValid());
     return mLayer->GetZoomAnimationId();