--- a/gfx/layers/apz/src/AsyncPanZoomController.cpp
+++ b/gfx/layers/apz/src/AsyncPanZoomController.cpp
@@ -2911,16 +2911,20 @@ void AsyncPanZoomController::AdjustScrol
   RequestContentRepaint();
   UpdateSharedCompositorFrameMetrics();
 }
 
 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);
   // We want to adjust the scroll offset such that the CSS point represented by aFocus remains
   // at the same position on the screen before and after the change in zoom. The below code
   // accomplishes this; see https://bugzilla.mozilla.org/show_bug.cgi?id=923431#c6 for an
   // in-depth explanation of how.
   mFrameMetrics.SetScrollOffset((mFrameMetrics.GetScrollOffset() + aFocus) - (aFocus / aScale));

--- a/gfx/layers/apz/src/AsyncPanZoomController.h
+++ b/gfx/layers/apz/src/AsyncPanZoomController.h
@@ -527,21 +527,29 @@ protected:
    * Helper method to cancel any gesture currently going to Gecko. Used
    * primarily when a user taps the screen over some clickable content but then
    * pans down instead of letting go (i.e. to cancel a previous touch so that a
    * new one can properly take effect.
    */
   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
    * point so that the page zooms inward/outward from that point.
    */
   void ScaleWithFocus(float aScale,
                       const CSSPoint& aFocus);
 
   /**