Before turning objects into spanners, check that they are not already spanners.

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 1173 matching lines @@
     LayoutBox& box = toLayoutBox(*object);
     if (s_couldContainSpanners) {
       // Remove all spanners (turn them into regular column content), as we can
       // no longer contain them.
       if (box.isColumnSpanAll()) {
         destroySpannerPlaceholder(box.spannerPlaceholder());
         continue;
       }
     } else if (descendantIsValidColumnSpanner(object)) {
       // We can now contain spanners, and we found a candidate. Turn it into a
-      // spanner.
-      createAndInsertSpannerPlaceholder(
-          &box, nextInPreOrderAfterChildrenSkippingOutOfFlow(this, &box));
+      // spanner, if it's not already one. We have to check if it's already a
+      // spanner, because in some cases we incorrectly think that we need to
+      // toggle spanners. One known case is when some ancestor changes
+      // writing-mode (which is an inherited property). Writing mode roots
+      // establish block formatting context (which means that there can be no
+      // column spanners inside). When changing the style on one object in the
+      // tree at a time, we're going to see writing mode roots that are not
+      // going to remain writing mode roots when all objects have been updated
+      // (because then all will have got the same writing mode).
+      if (!box.isColumnSpanAll()) {
+        createAndInsertSpannerPlaceholder(
+            &box, nextInPreOrderAfterChildrenSkippingOutOfFlow(this, &box));
+      }
       continue;
     }
     walkChildren = canContainSpannerInParentFragmentationContext(box);
   }
 }
 
 void LayoutMultiColumnFlowThread::computePreferredLogicalWidths() {
   LayoutFlowThread::computePreferredLogicalWidths();
 
   // The min/max intrinsic widths calculated really tell how much space elements
@@ skipping 117 matching lines @@
     const {
   return MultiColumnLayoutState(m_lastSetWorkedOn);
 }
 
 void LayoutMultiColumnFlowThread::restoreMultiColumnLayoutState(
     const MultiColumnLayoutState& state) {
   m_lastSetWorkedOn = state.columnSet();
 }
 
 }  // namespace blink