diff --git a/layout/base/Units.h b/layout/base/Units.h
index cae5ce4ada77..5aaaed8bd38a 100644
--- a/layout/base/Units.h
+++ b/layout/base/Units.h
@@ -106,20 +106,20 @@ struct CSSPixel {
   // Conversions to app units
 
   static nsPoint ToAppUnits(const CSSPoint& aPoint) {
-    return nsPoint(NSFloatPixelsToAppUnits(aPoint.x, float(nsDeviceContext::AppUnitsPerCSSPixel())),
-                   NSFloatPixelsToAppUnits(aPoint.y, float(nsDeviceContext::AppUnitsPerCSSPixel())));
+    return nsPoint(NSToCoordRoundWithClamp(aPoint.x * float(nsDeviceContext::AppUnitsPerCSSPixel())),
+                   NSToCoordRoundWithClamp(aPoint.y * float(nsDeviceContext::AppUnitsPerCSSPixel())));
   }
 
   static nsPoint ToAppUnits(const CSSIntPoint& aPoint) {
-    return nsPoint(NSIntPixelsToAppUnits(aPoint.x, nsDeviceContext::AppUnitsPerCSSPixel()),
-                   NSIntPixelsToAppUnits(aPoint.y, nsDeviceContext::AppUnitsPerCSSPixel()));
+    return nsPoint(NSToCoordRoundWithClamp(float(aPoint.x) * float(nsDeviceContext::AppUnitsPerCSSPixel())),
+                   NSToCoordRoundWithClamp(float(aPoint.y) * float(nsDeviceContext::AppUnitsPerCSSPixel())));
   }
 
   static nsRect ToAppUnits(const CSSRect& aRect) {
-    return nsRect(NSFloatPixelsToAppUnits(aRect.x, float(nsDeviceContext::AppUnitsPerCSSPixel())),
-                  NSFloatPixelsToAppUnits(aRect.y, float(nsDeviceContext::AppUnitsPerCSSPixel())),
-                  NSFloatPixelsToAppUnits(aRect.width, float(nsDeviceContext::AppUnitsPerCSSPixel())),
-                  NSFloatPixelsToAppUnits(aRect.height, float(nsDeviceContext::AppUnitsPerCSSPixel())));
+    return nsRect(NSToCoordRoundWithClamp(aRect.x * float(nsDeviceContext::AppUnitsPerCSSPixel())),
+                  NSToCoordRoundWithClamp(aRect.y * float(nsDeviceContext::AppUnitsPerCSSPixel())),
+                  NSToCoordRoundWithClamp(aRect.width * float(nsDeviceContext::AppUnitsPerCSSPixel())),
+                  NSToCoordRoundWithClamp(aRect.height * float(nsDeviceContext::AppUnitsPerCSSPixel())));
   }
 };
 
diff --git a/layout/base/nsPresContext.h b/layout/base/nsPresContext.h
index 979776472d8d..be8728cc5968 100644
--- a/layout/base/nsPresContext.h
+++ b/layout/base/nsPresContext.h
@@ -580,11 +580,11 @@ public:
   static int32_t AppUnitsPerCSSInch() { return nsDeviceContext::AppUnitsPerCSSInch(); }
 
   static nscoord CSSPixelsToAppUnits(int32_t aPixels)
-  { return NSIntPixelsToAppUnits(aPixels,
-                                 nsDeviceContext::AppUnitsPerCSSPixel()); }
+  { return NSToCoordRoundWithClamp(float(aPixels) *
+             float(nsDeviceContext::AppUnitsPerCSSPixel())); }
 
   static nscoord CSSPixelsToAppUnits(float aPixels)
-  { return NSFloatPixelsToAppUnits(aPixels,
+  { return NSToCoordRoundWithClamp(aPixels *
              float(nsDeviceContext::AppUnitsPerCSSPixel())); }
 
   static int32_t AppUnitsToIntCSSPixels(nscoord aAppUnits)