[css-grid] Fix positioned items with grid gaps

third_party/WebKit/Source/core/layout/LayoutGrid.cpp

 /*
  * Copyright (C) 2011 Apple Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 1519 matching lines @@
         else
             start = m_rowPositions[startLine] - m_rowPositions[0] + paddingBefore();
     }
 
     LayoutUnit end = isForColumns ? clientLogicalWidth() : clientLogicalHeight();
     if (!endIsAuto) {
         if (isForColumns)
             end = m_columnPositions[endLine] - m_columnPositions[0] + paddingStart();
         else
             end = m_rowPositions[endLine] - m_rowPositions[0] + paddingBefore();
+
+        // These vectors store line positions including gaps, but we shouldn't consider them for the edges of the grid.
+        if (endLine > firstExplicitLine && endLine < lastExplicitLine)
+            end -= guttersSize(direction, 2);
     }
 
     breadth = end - start;
     offset = start;
 
     if (isForColumns && !styleRef().isLeftToRightDirection() && !child.styleRef().hasStaticInlinePosition(child.isHorizontalWritingMode())) {
         // If the child doesn't have a static inline position (i.e. "left" and/or "right" aren't "auto",
         // we need to calculate the offset from the left (even if we're in RTL).
         if (endIsAuto) {
             offset = LayoutUnit();
         } else {
             LayoutUnit alignmentOffset =  m_columnPositions[0] - borderAndPaddingStart();
             LayoutUnit offsetFromLastLine = m_columnPositions[m_columnPositions.size() - 1] - m_columnPositions[endLine];
             offset = paddingLeft() +  alignmentOffset + offsetFromLastLine;
+
+            if (endLine > firstExplicitLine && endLine < lastExplicitLine)
+                offset += guttersSize(direction, 2);
         }
     }
 
     if (child.parent() == this && !startIsAuto) {
         // If column/row start is "auto" the static position has been already set in prepareChildForPositionedLayout().
         PaintLayer* childLayer = child.layer();
         if (isForColumns)
             childLayer->setStaticInlinePosition(borderStart() + offset);
         else
             childLayer->setStaticBlockPosition(borderBefore() + offset);
@@ skipping 488 matching lines @@
 
     return LayoutPoint(rowAxisOffset, columnAxisOffsetForChild(child, sizingData));
 }
 
 void LayoutGrid::paintChildren(const PaintInfo& paintInfo, const LayoutPoint& paintOffset) const
 {
     GridPainter(*this).paintChildren(paintInfo, paintOffset);
 }
 
 } // namespace blink