diff --git a/gfx/2d/Types.h b/gfx/2d/Types.h
index 7626987a87..ad83a79889 100644
--- a/gfx/2d/Types.h
+++ b/gfx/2d/Types.h
@@ -479,6 +479,16 @@ constexpr bool SideIsVertical(Side aSide)
   return aSide % 2;
 }
 
+/* @param aIsSecond: when true, return the clockwise second of the two
+ * corners associated with aSide. For example, with aSide = eSideBottom the
+ * result is eCornerBottomRight when aIsSecond is false, and
+ * eCornerBottomLeft when aIsSecond is true.
+ */
+constexpr Corner SideToFullCorner(Side aSide, bool aIsSecond)
+{
+  return Corner((aSide + aIsSecond) % 4);
+}
+
 } // namespace mozilla
 
 #endif /* MOZILLA_GFX_TYPES_H_ */
diff --git a/layout/style/nsStyleConsts.h b/layout/style/nsStyleConsts.h
index 879b77269d..96690ae2d8 100644
--- a/layout/style/nsStyleConsts.h
+++ b/layout/style/nsStyleConsts.h
@@ -20,8 +20,6 @@ namespace mozilla {
 // nsStyleCoord.cpp.
 // Arguments must not have side effects.
 
-#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)
 
diff --git a/layout/style/nsStyleCoord.cpp b/layout/style/nsStyleCoord.cpp
index ba673201e3..263b0844b2 100644
--- a/layout/style/nsStyleCoord.cpp
+++ b/layout/style/nsStyleCoord.cpp
@@ -380,10 +380,10 @@ CASE(eCornerBottomLeft,  false, eCornerBottomLeftX);
 CASE(eCornerBottomLeft,  true,  eCornerBottomLeftY);
 #undef CASE
 
-// Validation of NS_SIDE_TO_{FULL,HALF}_CORNER.
+// Validation of SideToFullCorner.
 #define CASE(side, second, result)                                            \
-  static_assert(NS_SIDE_TO_FULL_CORNER(side, second) == result,               \
-                "NS_SIDE_TO_FULL_CORNER is wrong")
+  static_assert(SideToFullCorner(side, second) == result,                     \
+                "SideToFullCorner is wrong")
 CASE(eSideTop,    false, eCornerTopLeft);
 CASE(eSideTop,    true,  eCornerTopRight);