Bug 1342571 - fix MaxStrokeExtents to account for partial pixel coverage properly. r=mstange

MozReview-Commit-ID: DS25AJKoVYo

gfx/2d/DrawCommand.h

@@ -372,6 +372,11 @@ PathExtentsToMaxStrokeExtents(const StrokeOptions &aStrokeOptions,
   double dx = styleExpansionFactor * hypot(aTransform._11, aTransform._21);
   double dy = styleExpansionFactor * hypot(aTransform._22, aTransform._12);
 
+  // Even if the stroke only partially covers a pixel, it must still render to
+  // full pixels. Round up to compensate for this.
+  dx = ceil(dx);
+  dy = ceil(dy);
+
   Rect result = aRect;
   result.Inflate(dx, dy);
   return result;

gfx/2d/PathHelpers.cpp

@@ -270,6 +270,12 @@ MaxStrokeExtents(const StrokeOptions& aStrokeOptions,
 
   double dx = styleExpansionFactor * hypot(aTransform._11, aTransform._21);
   double dy = styleExpansionFactor * hypot(aTransform._22, aTransform._12);
+
+  // Even if the stroke only partially covers a pixel, it must still render to
+  // full pixels. Round up to compensate for this.
+  dx = ceil(dx);
+  dy = ceil(dy);
+
   return Margin(dy, dx, dy, dx);
 }