[css-grid] Fix positioned items with grid gaps

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

Index: third_party/WebKit/Source/core/layout/LayoutGrid.cpp
diff --git a/third_party/WebKit/Source/core/layout/LayoutGrid.cpp b/third_party/WebKit/Source/core/layout/LayoutGrid.cpp
index ac6717df3c10aaeaf3018ac450a0efde495a3224..b80d0bfdbf7531ec067a96b3fe6444b87d93ec30 100644
--- a/third_party/WebKit/Source/core/layout/LayoutGrid.cpp
+++ b/third_party/WebKit/Source/core/layout/LayoutGrid.cpp
@@ -1537,6 +1537,10 @@ void LayoutGrid::offsetAndBreadthForPositionedChild(const LayoutBox& child, Grid
             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;
@@ -1551,6 +1555,9 @@ void LayoutGrid::offsetAndBreadthForPositionedChild(const LayoutBox& child, Grid
             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);
         }
     }