Paginated containers are opaque to enclosing fragmentation contexts.

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

 /*
  * Copyright (C) 2012 Apple Inc.  All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 418 matching lines @@
     spannerObjectInFlowThread->containingBlock()->setNeedsLayoutAndPrefWidthsRecalc(LayoutInvalidationReason::ColumnsChanged);
 
     // Now generate a column set for this ex-spanner, if needed and none is there for us already.
     flowThreadDescendantWasInserted(spannerObjectInFlowThread);
 
     return true;
 }
 
 LayoutMultiColumnFlowThread* LayoutMultiColumnFlowThread::enclosingFlowThread() const
 {
+    if (isLayoutPagedFlowThread()) {
+        // Paged overflow containers should never be fragmented by enclosing fragmentation
+        // contexts. They are to be treated as unbreakable content.
+        return nullptr;
+    }
     if (multiColumnBlockFlow()->isInsideFlowThread())
         return toLayoutMultiColumnFlowThread(locateFlowThreadContainingBlockOf(*multiColumnBlockFlow()));
     return nullptr;
 }
 
 FragmentationContext* LayoutMultiColumnFlowThread::enclosingFragmentationContext() const
 {
     if (LayoutMultiColumnFlowThread* enclosingFlowThread = this->enclosingFlowThread())
         return enclosingFlowThread;
     return view()->fragmentationContext();
@@ skipping 14 matching lines @@
         // We only insert additional fragmentainer groups in the initial layout pass. We only want
         // to balance columns in the last fragmentainer group (if we need to balance at all), so we
         // want that last fragmentainer group to be the same one in all layout passes that follow.
         return;
     }
 
     if (!columnSet->hasFragmentainerGroupForColumnAt(offsetInFlowThread)) {
         FragmentationContext* enclosingFragmentationContext = this->enclosingFragmentationContext();
         if (!enclosingFragmentationContext)
             return; // Not nested. We'll never need more rows than the one we already have then.
-
+        ASSERT(!isLayoutPagedFlowThread());
         // We have run out of columns here, so we add another row to hold more columns. When we add
         // a new row, it implicitly means that we're inserting another column in our enclosing
         // multicol container. That in turn may mean that we've run out of columns there too.
         const MultiColumnFragmentainerGroup& newRow = columnSet->appendNewFragmentainerGroup();
         if (LayoutMultiColumnFlowThread* enclosingFlowThread = enclosingFragmentationContext->associatedFlowThread())
             enclosingFlowThread->appendNewFragmentainerGroupIfNeeded(newRow.blockOffsetInEnclosingFragmentationContext());
     }
 }
 
 bool LayoutMultiColumnFlowThread::isFragmentainerLogicalHeightKnown()
@@ skipping 461 matching lines @@
     // the "mayBe"), if we're in an out-of-flow subtree and have an outer multicol container that
     // doesn't affect us, but that's okay. We'll discover that further down the road when trying to
     // locate our enclosing flow thread for real.
     bool mayBeNested = multiColumnBlockFlow()->isInsideFlowThread() || view()->fragmentationContext();
     if (!mayBeNested)
         return;
     appendNewFragmentainerGroupIfNeeded(logicalTopInFlowThreadAfterPagination);
 }
 
 }