--- a/gfx/2d/Types.h
+++ b/gfx/2d/Types.h
@@ -469,11 +469,16 @@ static inline HalfCorner operator++(Half
   return aHalfCorner;
 }
 
 constexpr bool HalfCornerIsX(HalfCorner aHalfCorner)
 {
   return !(aHalfCorner % 2);
 }
 
+constexpr Corner HalfToFullCorner(HalfCorner aHalfCorner)
+{
+  return Corner(aHalfCorner / 2);
+}
+
 } // namespace mozilla
 
 #endif /* MOZILLA_GFX_TYPES_H_ */

--- a/layout/style/nsStyleConsts.h
+++ b/layout/style/nsStyleConsts.h
@@ -15,17 +15,16 @@
 // XXX fold this into nsStyleContext and group by nsStyleXXX struct
 
 namespace mozilla {
 
 // The results of these conversion macros are exhaustively checked in
 // nsStyleCoord.cpp.
 // Arguments must not have side effects.
 
-#define NS_HALF_TO_FULL_CORNER(var_) ((var_)/2)
 #define NS_FULL_TO_HALF_CORNER(var_, vert_) ((var_)*2 + !!(vert_))
 
 #define NS_SIDE_IS_VERTICAL(side_) ((side_) % 2)
 #define NS_SIDE_TO_FULL_CORNER(side_, second_) \
   (((side_) + !!(second_)) % 4)
 #define NS_SIDE_TO_HALF_CORNER(side_, second_, parallel_) \
   ((((side_) + !!(second_))*2 + ((side_) + !(parallel_))%2) % 8)
 

--- a/layout/style/nsStyleCoord.cpp
+++ b/layout/style/nsStyleCoord.cpp
@@ -349,20 +349,20 @@ CASE(eCornerTopLeftY,     false);
 CASE(eCornerTopRightX,    true);
 CASE(eCornerTopRightY,    false);
 CASE(eCornerBottomRightX, true);
 CASE(eCornerBottomRightY, false);
 CASE(eCornerBottomLeftX,  true);
 CASE(eCornerBottomLeftY,  false);
 #undef CASE
 
-// Validation of NS_HALF_TO_FULL_CORNER.
+// Validation of HalfToFullCorner.
 #define CASE(corner, result)                                                  \
-  static_assert(NS_HALF_TO_FULL_CORNER(corner) == result,                 \
-                "NS_HALF_TO_FULL_CORNER is wrong")
+  static_assert(HalfToFullCorner(corner) == result,                           \
+                "HalfToFullCorner is wrong")
 CASE(eCornerTopLeftX,     eCornerTopLeft);
 CASE(eCornerTopLeftY,     eCornerTopLeft);
 CASE(eCornerTopRightX,    eCornerTopRight);
 CASE(eCornerTopRightY,    eCornerTopRight);
 CASE(eCornerBottomRightX, eCornerBottomRight);
 CASE(eCornerBottomRightY, eCornerBottomRight);
 CASE(eCornerBottomLeftX,  eCornerBottomLeft);
 CASE(eCornerBottomLeftY,  eCornerBottomLeft);