diff --git a/gfx/2d/BaseRect.h b/gfx/2d/BaseRect.h
index 459892defeb3..08094a4fbda0 100644
--- a/gfx/2d/BaseRect.h
+++ b/gfx/2d/BaseRect.h
@@ -441,18 +441,18 @@ struct BaseRect {
   }
 
   /**
-   * Clamp aRect to this rectangle. This returns aRect after it is forced
-   * inside the bounds of this rectangle. It will attempt to retain the size
-   * but will shrink the dimensions that don't fit.
+   * Clamp this rectangle to be inside aRect. The function returns a copy of
+   * this rect after it is forced inside the bounds of aRect. It will attempt to
+   * retain the size but will shrink the dimensions that don't fit.
    */
-  Sub ClampRect(const Sub& aRect) const
+  Sub ForceInside(const Sub& aRect) const
   {
     Sub rect(std::max(aRect.x, x),
              std::max(aRect.y, y),
              std::min(aRect.width, width),
              std::min(aRect.height, height));
-    rect.x = std::min(rect.XMost(), XMost()) - rect.width;
-    rect.y = std::min(rect.YMost(), YMost()) - rect.height;
+    rect.x = std::min(rect.XMost(), aRect.XMost()) - rect.width;
+    rect.y = std::min(rect.YMost(), aRect.YMost()) - rect.height;
     return rect;
   }
 
diff --git a/gfx/layers/ipc/AsyncPanZoomController.cpp b/gfx/layers/ipc/AsyncPanZoomController.cpp
index 789c4518a858..c5935e566026 100644
--- a/gfx/layers/ipc/AsyncPanZoomController.cpp
+++ b/gfx/layers/ipc/AsyncPanZoomController.cpp
@@ -1257,8 +1257,7 @@ const CSSRect AsyncPanZoomController::CalculatePendingDisplayPort(
     scrollOffset.y = scrollableRect.y;
   }
 
-  CSSRect shiftedDisplayPort = displayPort + scrollOffset;
-  return scrollableRect.ClampRect(shiftedDisplayPort) - scrollOffset;
+  return displayPort.ForceInside(scrollableRect - scrollOffset);
 }
 
 void AsyncPanZoomController::ScheduleComposite() {
diff --git a/layout/base/nsDisplayList.cpp b/layout/base/nsDisplayList.cpp
index cdddac27c088..57328ea7177f 100644
--- a/layout/base/nsDisplayList.cpp
+++ b/layout/base/nsDisplayList.cpp
@@ -716,7 +716,7 @@ static void RecordFrameMetrics(nsIFrame* aForFrame,
       ScreenIntRect screenBounds = ScreenIntRect::FromUnknownRect(mozilla::gfx::IntRect(
           bounds.x, bounds.y, bounds.width, bounds.height));
       AdjustForScrollBars(screenBounds, scrollableFrame);
-      metrics.mCompositionBounds = screenBounds.ClampRect(metrics.mCompositionBounds);
+      metrics.mCompositionBounds = metrics.mCompositionBounds.ForceInside(screenBounds);
       useWidgetBounds = true;
     }
   }