Consistent pagination strut propagation policies.

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

Index: third_party/WebKit/Source/core/layout/LayoutBlockFlow.cpp
diff --git a/third_party/WebKit/Source/core/layout/LayoutBlockFlow.cpp b/third_party/WebKit/Source/core/layout/LayoutBlockFlow.cpp
index 9a0a90f993fd03c929f4d54c7b9a85bba2ee6781..ef6aeb493e2048877e61076b897b8efaa4ad5b95 100644
--- a/third_party/WebKit/Source/core/layout/LayoutBlockFlow.cpp
+++ b/third_party/WebKit/Source/core/layout/LayoutBlockFlow.cpp
@@ -4124,10 +4124,20 @@ bool LayoutBlockFlow::allowsPaginationStrut() const {
   // If children are inline, allow the strut. We are probably a float.
   if (containingBlockFlow->childrenInline())
     return true;
-  // If this isn't the first in-flow object, there's a break opportunity before
-  // us, which means that we can allow the strut.
-  if (previousInFlowSiblingBox())
-    return true;
+  for (LayoutBox* sibling = previousSiblingBox(); sibling;
+       sibling = sibling->previousSiblingBox()) {
+    // What happens on the other side of a spanner is none of our concern, so
+    // stop here. Since there's no in-flow box between the previous spanner and
+    // us, there's no class A break point in front of us. We cannot even
+    // re-propagate pagination struts to our containing block, since the
+    // containing block starts in a different column row.
+    if (sibling->isColumnSpanAll())
+      return false;
+    // If this isn't the first in-flow object, there's a break opportunity
+    // before us, which means that we can allow the strut.
+    if (!sibling->isFloatingOrOutOfFlowPositioned())
+      return true;
+  }
   // This is a first in-flow child. We'll still allow the strut if it can be
   // re-propagated to our containing block.
   return containingBlockFlow->allowsPaginationStrut();