Fix LayoutTableRow::AddOverflowFromCell()

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

 /*
  * Copyright (C) 1997 Martin Jones (mjones@kde.org)
  *           (C) 1997 Torben Weis (weis@kde.org)
  *           (C) 1998 Waldo Bastian (bastian@kde.org)
  *           (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  * Copyright (C) 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2013
 *                Apple Inc.
  *               All rights reserved.
  *
@@ skipping 288 matching lines @@
   if (cell->RowSpan() == 1)
     return;
 
   // Cells only generates visual overflow.
   LayoutRect cell_visual_overflow_rect =
       cell->VisualOverflowRectForPropagation(StyleRef());
 
   // The cell and the row share the section's coordinate system. However
   // the visual overflow should be determined in the coordinate system of
   // the row, that's why we shift it below.
-  cell_visual_overflow_rect.MoveBy(-Location());

[Xianzhu, 2017/05/06 00:12:47]

cell_visual_overflow_rect is in cell's coordinates. To convert to row
coordinates, we should move by the difference of locations of the row and the
cell.

This was changed in
https://codereview.chromium.org/2786463004/diff/120001/third_party/WebKit/Sou...,
but the old version was also incorrect:

 LayoutUnit cellOffsetLogicalTopDifference =
    cell->location().y() - location().y();
 cellVisualOverflowRect.move(LayoutUnit(), cellOffsetLogicalTopDifference);	
 addContentsVisualOverflow(cellVisualOverflowRect);

Actually the above code should adjust by difference of x instead of y. The row
and the cell should have the same y, but different x.

+  cell_visual_overflow_rect.Move(cell->Location() - Location());
   AddContentsVisualOverflow(cell_visual_overflow_rect);
 
   // Table row paints its background behind cells. If the cell spans multiple
   // rows, the row's visual rect should be expanded to cover the cell.
   if (StyleRef().HasBackground()) {
     LayoutRect cell_background_rect = cell->FrameRect();
     cell_background_rect.MoveBy(-Location());
     AddSelfVisualOverflow(cell_background_rect);
   }
 }
 
 bool LayoutTableRow::IsFirstRowInSectionAfterHeader() const {
   // If there isn't room on the page for at least one content row after the
   // header group, then we won't repeat the header on each page.
   // https://drafts.csswg.org/css-tables-3/#repeated-headers reads like
   // it wants us to drop headers on only the pages that a single row
   // won't fit but we avoid the complexity of that reading until it
   // is clarified. Tracked by crbug.com/675904
   if (RowIndex())
     return false;
   LayoutTableSection* header = Table()->Header();
   return header && Table()->SectionAbove(Section()) == header &&
          header->GetPaginationBreakability() != kAllowAnyBreaks;
 }
 
 }  // namespace blink