Begin tracking why layout is invalidated

Source/core/paint/DeprecatedPaintLayerScrollableArea.cpp

 /*
  * Copyright (C) 2006, 2007, 2008, 2009, 2010, 2011, 2012 Apple Inc. All rights reserved.
  *
  * Portions are Copyright (C) 1998 Netscape Communications Corporation.
  *
  * Other contributors:
  *   Robert O'Callahan <roc+@cs.cmu.edu>
  *   David Baron <dbaron@fas.harvard.edu>
  *   Christian Biesinger <cbiesinger@web.de>
  *   Randall Jesup <rjesup@wgate.com>
@@ skipping 823 matching lines @@
     if (Scrollbar* verticalScrollbar = this->verticalScrollbar()) {
         int clientHeight = box().pixelSnappedClientHeight();
         verticalScrollbar->setProportion(clientHeight, overflowRect().height());
     }
 
     bool hasHorizontalOverflow = this->hasHorizontalOverflow();
     bool hasVerticalOverflow = this->hasVerticalOverflow();
     bool autoHorizontalScrollBarChanged = box().hasAutoHorizontalScrollbar() && (hasHorizontalScrollbar() != hasHorizontalOverflow);
     bool autoVerticalScrollBarChanged = box().hasAutoVerticalScrollbar() && (hasVerticalScrollbar() != hasVerticalOverflow);
     if (autoHorizontalScrollBarChanged || autoVerticalScrollBarChanged)
-        box().setNeedsLayoutAndFullPaintInvalidation();
+        box().setNeedsLayoutAndFullPaintInvalidation(LayoutInvalidationReason::Unknown);

[esprehn, 2015/03/23 20:19:44]

OverflowChanged or better ScrollbarsChanged

 }
 
 DoubleSize DeprecatedPaintLayerScrollableArea::clampScrollOffset(const DoubleSize& scrollOffset) const
 {
     int maxX = scrollWidth() - box().pixelSnappedClientWidth();
     int maxY = scrollHeight() - box().pixelSnappedClientHeight();
 
     double x = std::max(std::min(scrollOffset.width(), static_cast<double>(maxX)), 0.0);
     double y = std::max(std::min(scrollOffset.height(), static_cast<double>(maxY)), 0.0);
     return DoubleSize(x, y);
@@ skipping 531 matching lines @@
 void DeprecatedPaintLayerScrollableArea::setTopmostScrollChild(DeprecatedPaintLayer* scrollChild)
 {
     // We only want to track the topmost scroll child for scrollable areas with
     // overlay scrollbars.
     if (!hasOverlayScrollbars())
         return;
     m_nextTopmostScrollChild = scrollChild;
 }
 
 } // namespace blink