Use LayoutObject to locate the flow thread.

Source/core/paint/DeprecatedPaintLayer.cpp

Index: Source/core/paint/DeprecatedPaintLayer.cpp
diff --git a/Source/core/paint/DeprecatedPaintLayer.cpp b/Source/core/paint/DeprecatedPaintLayer.cpp
index 8167ca351f47d5c7b89ad8c07e566e66749ae215..ace277e51b2c0c2cd58f39e821bd18650987fc31 100644
--- a/Source/core/paint/DeprecatedPaintLayer.cpp
+++ b/Source/core/paint/DeprecatedPaintLayer.cpp
@@ -476,37 +476,17 @@ void DeprecatedPaintLayer::updatePagination()
         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()