diff --git a/gfx/layers/apz/src/AsyncPanZoomController.cpp b/gfx/layers/apz/src/AsyncPanZoomController.cpp
index ad563fd834d4..f45a3746312f 100644
--- a/gfx/layers/apz/src/AsyncPanZoomController.cpp
+++ b/gfx/layers/apz/src/AsyncPanZoomController.cpp
@@ -2916,6 +2916,10 @@ void AsyncPanZoomController::ScrollBy(const CSSPoint& aOffset) {
   mFrameMetrics.ScrollBy(aOffset);
 }
 
+void AsyncPanZoomController::ScrollByAndClamp(const CSSPoint& aOffset) {
+  mFrameMetrics.ClampAndSetScrollOffset(mFrameMetrics.GetScrollOffset() + aOffset);
+}
+
 void AsyncPanZoomController::ScaleWithFocus(float aScale,
                                             const CSSPoint& aFocus) {
   mFrameMetrics.ZoomBy(aScale);
diff --git a/gfx/layers/apz/src/AsyncPanZoomController.h b/gfx/layers/apz/src/AsyncPanZoomController.h
index 3272c200caf9..c984bba089e8 100644
--- a/gfx/layers/apz/src/AsyncPanZoomController.h
+++ b/gfx/layers/apz/src/AsyncPanZoomController.h
@@ -532,10 +532,18 @@ protected:
   nsEventStatus OnCancelTap(const TapGestureInput& aEvent);
 
   /**
-   * Scrolls the viewport by an X,Y offset.
+   * Scroll the scroll frame by an X,Y offset.
+   * The resulting scroll offset is not clamped to the scrollable rect;
+   * the caller must ensure it stays within range.
    */
   void ScrollBy(const CSSPoint& aOffset);
 
+  /**
+   * Scroll the scroll frame by an X,Y offset, clamping the resulting
+   * scroll offset to the scrollable rect.
+   */
+  void ScrollByAndClamp(const CSSPoint& aOffset);
+
   /**
    * Scales the viewport by an amount (note that it multiplies this scale in to
    * the current scale, it doesn't set it to |aScale|). Also considers a focus