Look inside inner nested multicols to calculate minimum space shortage.

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

Index: third_party/WebKit/Source/core/layout/ColumnBalancer.cpp
diff --git a/third_party/WebKit/Source/core/layout/ColumnBalancer.cpp b/third_party/WebKit/Source/core/layout/ColumnBalancer.cpp
index 3de1e264d6c82d0d4b2cccf9a02f19e79ed3eb83..ffabb59fe7cd7ae8e63a0171016ee8bd4d6701a0 100644
--- a/third_party/WebKit/Source/core/layout/ColumnBalancer.cpp
+++ b/third_party/WebKit/Source/core/layout/ColumnBalancer.cpp
@@ -6,6 +6,7 @@
 
 #include "core/layout/ColumnBalancer.h"
 
+#include "core/layout/LayoutMultiColumnFlowThread.h"
 #include "core/layout/LayoutMultiColumnSet.h"
 
 namespace blink {
@@ -273,6 +274,19 @@ void MinimumSpaceShortageFinder::examineBoxAfterEntering(const LayoutBox& box)
             recordSpaceShortage(spaceUsedInLastColumn);
         }
     }
+
+    // If this is an inner multicol container, look for space shortage inside it.
+    if (!box.isLayoutBlockFlow())
+        return;
+    LayoutMultiColumnFlowThread* flowThread = toLayoutBlockFlow(box).multiColumnFlowThread();
+    if (!flowThread)
+        return;
+    for (const LayoutMultiColumnSet* columnSet = flowThread->firstMultiColumnSet(); columnSet; columnSet = columnSet->nextSiblingMultiColumnSet()) {
+        for (const MultiColumnFragmentainerGroup& row : columnSet->fragmentainerGroups()) {
+            MinimumSpaceShortageFinder innerFinder(row);
+            recordSpaceShortage(innerFinder.minimumSpaceShortage());
+        }
+    }
 }
 
 void MinimumSpaceShortageFinder::examineBoxBeforeLeaving(const LayoutBox& box)