Fix spanning cell painting background from wrong row

third_party/WebKit/Source/core/layout/LayoutTableSection.h

Index: third_party/WebKit/Source/core/layout/LayoutTableSection.h
diff --git a/third_party/WebKit/Source/core/layout/LayoutTableSection.h b/third_party/WebKit/Source/core/layout/LayoutTableSection.h
index bf276c708b87020816f1e2911120ae70b0e20ad0..53472bd30a77a144a5db76c0f82e5f6e56f43bc0 100644
--- a/third_party/WebKit/Source/core/layout/LayoutTableSection.h
+++ b/third_party/WebKit/Source/core/layout/LayoutTableSection.h
@@ -141,7 +141,9 @@ class CORE_EXPORT LayoutTableSection final : public LayoutTableBoxComponent {
     // This is the cell in the grid "slot" that is on top of the others
     // (aka the last cell in DOM order for this slot).
     //
-    // This is the cell originating from this slot if it exists.
+    // Multiple grid slots can have the same primary cell if the cell spans
+    // into the grid slots. The slot having the smallest row index and
+    // smallest effective column index is the originating slot of the cell.
     //
     // The concept of a primary cell is dubious at most as it doesn't
     // correspond to a DOM or rendering concept. Also callers should be
@@ -229,6 +231,21 @@ class CORE_EXPORT LayoutTableSection final : public LayoutTableBoxComponent {
         row, effectiveColumn);
   }
 
+  // Returns the primary cell at (row, effectiveColumn) if the cell exists and
+  // originates from (instead of spanning into) the grid slot, or nullptr.
+  LayoutTableCell* originatingCellAt(unsigned row, unsigned effectiveColumn) {

[wkorman, 2017/03/29 18:12:33]

Do these methods have to be implemented in header? Prefer putting in .cpp if we
can for faster build time when modifying.

[Xianzhu, 2017/03/29 18:44:41]

Done.

+    LayoutTableCell* cell = primaryCellAt(row, effectiveColumn);
+    if ((!row || primaryCellAt(row - 1, effectiveColumn) != cell) &&
+        (!effectiveColumn || primaryCellAt(row, effectiveColumn - 1) != cell))
+      return cell;
+    return nullptr;
+  }
+  const LayoutTableCell* originatingCellAt(unsigned row,
+                                           unsigned effectiveColumn) const {
+    return const_cast<LayoutTableSection*>(this)->originatingCellAt(

[wkorman, 2017/03/29 18:12:33]

Why do we need the const_cast and the separate method? Is primaryCellAt not
const?

[Xianzhu, 2017/03/29 18:44:41]

This follows the strict-const style which is used in primaryCell etc., like:

  // non const version.
  Something* method() { ... }
  // const version.
  const SomeThing* method() const { return const_cast<Class*>(this)->method(); }

I usually use this style for new methods if the sibling methods are already in
this style.

Otherwise I would use the loose-const style:
  Something* method() const { ... }

[Xianzhu, 2017/03/29 18:49:12]

As only the const version is used, I will remove the non-const one.

+        row, effectiveColumn);
+  }
+
   unsigned numCols(unsigned row) const { return m_grid[row].row.size(); }
 
   // Returns null for cells with a rowspan that exceed the last row. Possibly
@@ -300,8 +317,11 @@ class CORE_EXPORT LayoutTableSection final : public LayoutTableBoxComponent {
   // columnPos vectors.
   LayoutRect logicalRectForWritingModeAndDirection(const LayoutRect&) const;
 
+  // Returns a row or column span covering all grid slots from each of which
+  // a primary cell intersecting visualRect originates.

[wkorman, 2017/03/29 18:12:33]

|visualRect|

[Xianzhu, 2017/03/29 18:44:41]

Done.

   CellSpan dirtiedRows(const LayoutRect& visualRect) const;
   CellSpan dirtiedEffectiveColumns(const LayoutRect& visualRect) const;
+
   const HashSet<LayoutTableCell*>& overflowingCells() const {
     return m_overflowingCells;
   }
@@ -475,6 +495,9 @@ class CORE_EXPORT LayoutTableSection final : public LayoutTableBoxComponent {
   // The use is to disable a painting optimization where we just paint the
   // invalidated cells.
   bool m_hasMultipleCellLevels;
+
+  // This is set if there is any cell spanning multiple rows or cols.

[wkorman, 2017/03/29 18:12:33]

Slight rephrase for clarity, something like, "Whether any cell spans multiple
rows or cols."
since one could set it to false and it would still have been "set".

[Xianzhu, 2017/03/29 18:44:41]

Done.

+  bool m_hasSpanningCells;
 };
 
 DEFINE_LAYOUT_OBJECT_TYPE_CASTS(LayoutTableSection, isTableSection());