Compute outline mid-line in way that works with negative start-values

Source/core/paint/ObjectPainter.cpp

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "config.h"
 #include "core/paint/ObjectPainter.h"
 
 #include "core/layout/LayoutInline.h"
 #include "core/layout/LayoutObject.h"
 #include "core/layout/LayoutTheme.h"
@@ skipping 324 matching lines @@
 
     bool wasAntialiased = graphicsContext->shouldAntialias();
     StrokeStyle oldStrokeStyle = graphicsContext->strokeStyle();
     graphicsContext->setShouldAntialias(antialias);
     graphicsContext->setStrokeColor(color);
     graphicsContext->setStrokeThickness(thickness);
     graphicsContext->setStrokeStyle(style == DASHED ? DashedStroke : DottedStroke);
 
     switch (side) {
     case BSBottom:
-    case BSTop:
-        graphicsContext->drawLine(IntPoint(x1, (y1 + y2) / 2), IntPoint(x2, (y1 + y2) / 2));
+    case BSTop: {
+        int midY = y1 + thickness / 2;
+        graphicsContext->drawLine(IntPoint(x1, midY), IntPoint(x2, midY));
         break;
+    }
     case BSRight:
-    case BSLeft:
-        graphicsContext->drawLine(IntPoint((x1 + x2) / 2, y1), IntPoint((x1 + x2) / 2, y2));
+    case BSLeft: {
+        int midX = x1 + thickness / 2;
+        graphicsContext->drawLine(IntPoint(midX, y1), IntPoint(midX, y2));
         break;
     }
+    }
     graphicsContext->setShouldAntialias(wasAntialiased);
     graphicsContext->setStrokeStyle(oldStrokeStyle);
 }
 
 void ObjectPainter::drawDoubleBoxSide(GraphicsContext* graphicsContext, int x1, int y1, int x2, int y2,
     int length, BoxSide side, Color color, int thickness, int adjacentWidth1, int adjacentWidth2, bool antialias)
 {
     int thirdOfThickness = (thickness + 1) / 3;
     ASSERT(thirdOfThickness);
 
@@ skipping 151 matching lines @@
         quad[1] = FloatPoint(x1, y2 - std::max(adjacentWidth2, 0));
         quad[2] = FloatPoint(x2, y2 - std::max(-adjacentWidth2, 0));
         quad[3] = FloatPoint(x2, y1 + std::max(-adjacentWidth1, 0));
         break;
     }
 
     graphicsContext->fillPolygon(4, quad, color, antialias);
 }
 
 } // namespace blink