Initialize child framesets when they become part of the parent frameset grid.

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

 /**
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 2000 Simon Hausmann <hausmann@kde.org>
  *           (C) 2000 Stefan Schimanski (1Stein@gmx.de)
  * Copyright (C) 2004, 2005, 2006 Apple Computer, Inc.
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
  * License as published by the Free Software Foundation; either
  * version 2 of the License, or (at your option) any later version.
@@ skipping 400 matching lines @@
     int borderThickness = frameSet()->border();
     LayoutSize size;
     LayoutPoint position;
     for (int r = 0; r < rows; r++) {
         position.setX(LayoutUnit());
         size.setHeight(LayoutUnit(m_rows.m_sizes[r]));
         for (int c = 0; c < cols; c++) {
             child->setLocation(position);
             size.setWidth(LayoutUnit(m_cols.m_sizes[c]));
 
-            // has to be resized and itself resize its contents
-            if (size != child->size()) {
+            // If we have a new size, we need to resize and layout the child. If the size is 0x0 we
+            // also need to lay out, since this may mean that we're dealing with a child frameset
+            // that wasn't previously initialized properly, because it was previously hidden, but
+            // no longer is, because rows * cols may have increased.
+            if (size != child->size() || size.isEmpty()) {

[mstensho (USE GERRIT), 2016/04/01 12:21:47]

This is the simplest fix that I can think of. A safer but slightly uglier fix
could be to condition the child on being a LayoutFrameSet without rows() and
cols() initialized. But hopefully what I'm doing here doesn't have any unwanted
side-effects (performance-wise or other-wise)?

                 child->setSize(size);
                 child->setNeedsLayoutAndFullPaintInvalidation(LayoutInvalidationReason::SizeChanged);
                 child->layout();
             }
 
             position.setX(position.x() + size.width() + borderThickness);
 
             child = child->nextSiblingBox();
             if (!child)
                 return;
@@ skipping 135 matching lines @@
         return SetCursor;
     }
     if (canResizeColumn(roundedPoint)) {
         cursor = columnResizeCursor();
         return SetCursor;
     }
     return LayoutBox::getCursor(point, cursor);
 }
 
 } // namespace blink