Col width is not honored when dynamically updated and it would not make table narrower

Source/core/rendering/RenderTableCol.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, 2009 Apple Inc. All rights reserved.
  * Copyright (C) 2008 Nokia Corporation and/or its subsidiary(-ies)
  *
  * This library is free software; you can redistribute it and/or
@@ skipping 33 matching lines @@
     updateFromElement();
 }
 
 void RenderTableCol::styleDidChange(StyleDifference diff, const RenderStyle* oldStyle)
 {
     RenderBox::styleDidChange(diff, oldStyle);
 
     // If border was changed, notify table.
     if (parent()) {
         RenderTable* table = this->table();
-        if (table && !table->selfNeedsLayout() && !table->normalChildNeedsLayout() && oldStyle && oldStyle->border() != style()->border())
+        if (table && !table->selfNeedsLayout() && !table->normalChildNeedsLayout() && oldStyle && oldStyle->border() != style()->border()) {
             table->invalidateCollapsedBorders();
+        } else if (oldStyle && oldStyle->logicalWidth() != style()->logicalWidth()) {
+            unsigned nEffCols = table->numEffCols();
+            unsigned currentColumn = 0;
+            for (RenderTableCol* column = table->firstColumn()->nextColumn(); column; column = column->nextColumn()) {
+                if (this == column)
+                    break;
+                currentColumn += column->span();
+            }

[Julien - ping for review, 2014/03/06 00:54:47]

This span computation code actually doesn't work, totally missed that in
previous reviews. It only works because both test cases have the exact same
layout:
<colgroup>
   <col></col>
</colgroup>

The following are allowed too and will be mishandled by your code:

<colgroup span="5"/>
<col/>

or

<colgroup>
   <col span="3"/>
   <col/>
</colgroup>

The rule is that <colgroup> is only allowed to have a |span| attribute if it
doesn't have <col> children (see HTML5
http://www.whatwg.org/specs/web-apps/current-work/multipage/tabular-data.html...).

I don't think we have some code to do this computation and I fear that we don't
want to spread such tricky code around (RenderTable already has some column
information but we can't reuse it). My take is to revert back to the original
approach to invalidate all the cells but put a FIXME that explains that we could
do better.

The previous cases should be tested as they would have been broken by this
change.

+            for (RenderTableSection* section = table->topSection(); section; section = table->sectionBelow(section)) {
+                unsigned currentColSpan = currentColumn + span();
+                for (unsigned j = currentColumn; j < currentColSpan && currentColSpan <= nEffCols; j++) {
+                    for (unsigned i = 0; i < section->numRows(); i++) {
+                        RenderTableCell* cell = section->primaryCellAt(i, j);
+                        if (!cell)
+                            continue;
+                        cell->setPreferredLogicalWidthsDirty();
+                    }
+                }
+            }
+        }
     }
 }
 
 void RenderTableCol::updateFromElement()
 {
     unsigned oldSpan = m_span;
     Node* n = node();
     if (n && (n->hasTagName(colTag) || n->hasTagName(colgroupTag))) {
         HTMLTableColElement* tc = toHTMLTableColElement(n);
         m_span = tc->span();
@@ skipping 108 matching lines @@
     return style()->borderStart();
 }
 
 const BorderValue& RenderTableCol::borderAdjoiningCellAfter(const RenderTableCell* cell) const
 {
     ASSERT_UNUSED(cell, table()->colElement(cell->col() - 1) == this);
     return style()->borderEnd();
 }
 
 }