diff --git a/gfx/layers/apz/src/APZCTreeManager.cpp b/gfx/layers/apz/src/APZCTreeManager.cpp
index 44e2b342490c..3033d516863b 100644
--- a/gfx/layers/apz/src/APZCTreeManager.cpp
+++ b/gfx/layers/apz/src/APZCTreeManager.cpp
@@ -3357,6 +3357,13 @@ void APZCTreeManager::SendSubtreeTransformsToChromeMainThread(
   controller->NotifyLayerTransforms(messages);
 }
 
+void APZCTreeManager::SetFixedLayerMargins(ScreenIntCoord aTop,
+                                           ScreenIntCoord aBottom) {
+  RecursiveMutexAutoLock lock(mTreeLock);
+  mFixedLayerMargins.top = aTop;
+  mFixedLayerMargins.bottom = aBottom;
+}
+
 /*static*/
 LayerToParentLayerMatrix4x4 APZCTreeManager::ComputeTransformForScrollThumb(
     const LayerToParentLayerMatrix4x4& aCurrentTransform,
diff --git a/gfx/layers/apz/src/APZCTreeManager.h b/gfx/layers/apz/src/APZCTreeManager.h
index cebed56ebd10..255bb0bd30c9 100644
--- a/gfx/layers/apz/src/APZCTreeManager.h
+++ b/gfx/layers/apz/src/APZCTreeManager.h
@@ -455,6 +455,11 @@ class APZCTreeManager : public IAPZCTreeManager, public APZInputBridge {
   void SendSubtreeTransformsToChromeMainThread(
       const AsyncPanZoomController* aAncestor);
 
+  /**
+   * Set fixed layer margins for dynamic toolbar.
+   */
+  void SetFixedLayerMargins(ScreenIntCoord aTop, ScreenIntCoord aBottom);
+
   /**
    * Compute the updated shadow transform for a scroll thumb layer that
    * reflects async scrolling of the associated scroll frame.
@@ -863,6 +868,11 @@ class APZCTreeManager : public IAPZCTreeManager, public APZInputBridge {
   /* Stores the current mouse position in screen coordinates.
    */
   ScreenPoint mCurrentMousePosition;
+  /* Extra margins that should be applied to content that fixed wrt. the
+   * RCD-RSF, to account for the dynamic toolbar.
+   * Acquire mTreeLock before accessing this.
+   */
+  ScreenMargin mFixedLayerMargins;
   /* For logging the APZC tree for debugging (enabled by the apz.printtree
    * pref). */
   gfx::TreeLog<gfx::LOG_DEFAULT> mApzcTreeLog;
diff --git a/gfx/layers/ipc/CompositorBridgeParent.cpp b/gfx/layers/ipc/CompositorBridgeParent.cpp
index f95a09b21ac1..503455fe6c54 100644
--- a/gfx/layers/ipc/CompositorBridgeParent.cpp
+++ b/gfx/layers/ipc/CompositorBridgeParent.cpp
@@ -1442,6 +1442,10 @@ void CompositorBridgeParent::SetFixedLayerMargins(ScreenIntCoord aTop,
     manager->SetFixedLayerMargins(aTop, aBottom);
   }
 
+  if (mApzcTreeManager) {
+    mApzcTreeManager->SetFixedLayerMargins(aTop, aBottom);
+  }
+
   Invalidate();
   ScheduleComposition();
 }