Respect constrained height on nested multicol containers.

third_party/WebKit/Source/core/layout/LayoutMultiColumnSet.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 222 matching lines @@
 LayoutMultiColumnSet* LayoutMultiColumnSet::previousSiblingMultiColumnSet()
     const {
   for (LayoutObject* sibling = previousSibling(); sibling;
        sibling = sibling->previousSibling()) {
     if (sibling->isLayoutMultiColumnSet())
       return toLayoutMultiColumnSet(sibling);
   }
   return nullptr;
 }
 
-bool LayoutMultiColumnSet::hasFragmentainerGroupForColumnAt(
+bool LayoutMultiColumnSet::needsNewFragmentainerGroupAt(
     LayoutUnit offsetInFlowThread,
     PageBoundaryRule pageBoundaryRule) const {
+  // First the cheap check: Perhaps the last fragmentainer group has sufficient
+  // capacity?
   const MultiColumnFragmentainerGroup& lastRow = lastFragmentainerGroup();
   LayoutUnit maxLogicalBottomInFlowThread =
       lastRow.logicalTopInFlowThread() + fragmentainerGroupCapacity(lastRow);
-  if (pageBoundaryRule == AssociateWithFormerPage)
-    return offsetInFlowThread <= maxLogicalBottomInFlowThread;
-  return offsetInFlowThread < maxLogicalBottomInFlowThread;
+  if (pageBoundaryRule == AssociateWithFormerPage) {
+    if (offsetInFlowThread <= maxLogicalBottomInFlowThread)
+      return false;
+  } else if (offsetInFlowThread < maxLogicalBottomInFlowThread) {
+    return false;
+  }
+
+  // So, there's not enough room in the last fragmentainer group. However,
+  // there can only ever be one fragmentainer group per column set if we're not
+  // nested inside another fragmentation context. We'll just create overflowing
+  // columns if the fragmentainer group cannot hold all the content.
+  if (!multiColumnFlowThread()->enclosingFragmentationContext())
+    return false;
+
+  // We're in a nested fragmentation context, and the last fragmentainer group
+  // cannot hold content at the specified offset without overflowing. This
+  // usually warrants a new fragmentainer group; however, this will not be the
+  // case if we have already allocated all available block space in this
+  // multicol container. When setting up a new fragmentainer group, we always
+  // constrain against the remaining portion of any specified
+  // height/max-height. This means that we shouldn't allow creation of
+  // fragmentainer groups below the bottom of the multicol container, or we'd
+  // end up with zero-height fragmentainer groups (or actually 1px; see
+  // heightAdjustedForRowOffset() in MultiColumnFragmentainerGroup, which
+  // guards against zero-height groups), i.e. potentially a lot of pretty
+  // useless fragmentainer groups, and possibly broken layout too. Instead,
+  // we'll just allow additional (overflowing) columns to be created in the
+  // last fragmentainer group, similar to what we do when we're not nested.
+  LayoutUnit logicalBottom = lastRow.logicalTop() + lastRow.logicalHeight();
+  LayoutUnit spaceUsed = logicalBottom + logicalTopFromMulticolContentEdge();
+  LayoutUnit maxColumnHeight = multiColumnFlowThread()->maxColumnLogicalHeight();
+  return maxColumnHeight - spaceUsed > LayoutUnit();
 }
 
 MultiColumnFragmentainerGroup&
 LayoutMultiColumnSet::appendNewFragmentainerGroup() {
   MultiColumnFragmentainerGroup newGroup(*this);
   {  // Extra scope here for previousGroup; it's potentially invalid once we
      // modify the m_fragmentainerGroups Vector.
     MultiColumnFragmentainerGroup& previousGroup = m_fragmentainerGroups.last();
 
     // This is the flow thread block offset where |previousGroup| ends and
@@ skipping 321 matching lines @@
   // Now add in column rule bounds, if present.
   Vector<LayoutRect> columnRuleBounds;
   if (computeColumnRuleBounds(LayoutPoint(), columnRuleBounds)) {
     for (auto& bound : columnRuleBounds)
       blockFlowBounds.unite(bound);
   }
   return blockFlowBounds;
 }
 
 }  // namespace blink