Use LayoutObject to locate the flow thread.

Source/core/paint/DeprecatedPaintLayer.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 458 matching lines @@
     // Each paginated layer has to paint on its own. There is no recurring into child layers. Each
     // layer has to be checked individually and genuinely know if it is going to have to split
     // itself up when painting only its contents (and not any other descendant layers). We track an
     // enclosingPaginationLayer instead of using a simple bit, since we want to be able to get back
     // to that layer easily.
     if (layoutObject()->isLayoutFlowThread()) {
         m_enclosingPaginationLayer = this;
         return;
     }
 
-    if (!m_stackingNode->isTreatedAsStackingContextForPainting()) {
-        // We cannot take the fast path for spanners, as they do not have their nearest ancestor
-        // pagination layer (flow thread) in their containing block chain.
-        if (!layoutObject()->isColumnSpanAll()) {
-            // Content inside a transform is not considered to be paginated, since we simply
-            // paint the transform multiple times in each column, so we don't have to use
-            // fragments for the transformed content.
-            m_enclosingPaginationLayer = parent()->enclosingPaginationLayer();
-            if (m_enclosingPaginationLayer && m_enclosingPaginationLayer->hasTransformRelatedProperty())
-                m_enclosingPaginationLayer = 0;
-            return;
-        }
+    // Unless we're a spanner or establish a stacking context, we can take the fast path: Just use
+    // the same pagination layer as our parent. Spanners cannot do this, because they don't have
+    // that pagination layer in their containing block chain.
+    if (!m_stackingNode->isTreatedAsStackingContextForPainting() && !layoutObject()->isColumnSpanAll()) {

[chrishtr, 2015/07/14 18:54:01]

Is this fast path needed for correctness?

Also, you mention the flow thread cannot be transformed. Is there an explicit
layout test that it cannot?

[mstensho (USE GERRIT), 2015/07/14 19:13:39]

No, I don't think so. I really hope not. :) I realize now that we actually hit
the fast path very rarely, isTreatedAsStackingContextForPainting() meant
something other than what I thought (it includes anything non-statically
positioned). Maybe just kill it, then?
 
A flow thread is an anonymous layout object, and there's no way CSS can specify
a transform on it.

[chrishtr, 2015/07/14 19:46:44]

Let's kill it.
Ah right.

[mstensho (USE GERRIT), 2015/07/14 19:52:34]

Done.

+        m_enclosingPaginationLayer = parent()->enclosingPaginationLayer();
+        return;
     }
 
-    // Walk up our containing block chain looking for an enclosing layer. Once we find one, then we
-    // just check its pagination status.
-    LayoutView* view = layoutObject()->view();
-    LayoutBlock* containingBlock;
-    for (containingBlock = layoutObject()->containingBlock();
-        containingBlock && containingBlock != view;
-        containingBlock = containingBlock->containingBlock()) {
-        if (containingBlock->hasLayer()) {
-            // Content inside a transform is not considered to be paginated, since we simply
-            // paint the transform multiple times in each column, so we don't have to use
-            // fragments for the transformed content.
-            m_enclosingPaginationLayer = containingBlock->layer()->enclosingPaginationLayer();
-            if (m_enclosingPaginationLayer && m_enclosingPaginationLayer->hasTransformRelatedProperty())
-                m_enclosingPaginationLayer = 0;
-            return;
-        }
-    }
+    // Use the layout tree to find our enclosing pagination layer.
+    if (LayoutFlowThread* containingFlowThread = layoutObject()->flowThreadContainingBlock())
+        m_enclosingPaginationLayer = containingFlowThread->layer();
 }
 
 void DeprecatedPaintLayer::clearPaginationRecursive()
 {
     m_enclosingPaginationLayer = 0;
     for (DeprecatedPaintLayer* child = firstChild(); child; child = child->nextSibling())
         child->clearPaginationRecursive();
 }
 
 LayoutPoint DeprecatedPaintLayer::positionFromPaintInvalidationBacking(const LayoutObject* layoutObject, const LayoutBoxModelObject* paintInvalidationContainer, const PaintInvalidationState* paintInvalidationState)
@@ skipping 2221 matching lines @@
 
 void showLayerTree(const blink::LayoutObject* layoutObject)
 {
     if (!layoutObject) {
         fprintf(stderr, "Cannot showLayerTree. Root is (nil)\n");
         return;
     }
     showLayerTree(layoutObject->enclosingLayer());
 }
 #endif