Respect constrained height on nested multicol containers.

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

Index: third_party/WebKit/Source/core/layout/LayoutMultiColumnSet.cpp
diff --git a/third_party/WebKit/Source/core/layout/LayoutMultiColumnSet.cpp b/third_party/WebKit/Source/core/layout/LayoutMultiColumnSet.cpp
index 1539e3fb17c15115155bea3bd9635d8078785f4f..8cfe38d6d8800e6bba06d823eea2aae82ca76975 100644
--- a/third_party/WebKit/Source/core/layout/LayoutMultiColumnSet.cpp
+++ b/third_party/WebKit/Source/core/layout/LayoutMultiColumnSet.cpp
@@ -240,15 +240,46 @@ LayoutMultiColumnSet* LayoutMultiColumnSet::previousSiblingMultiColumnSet()
   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&