An object may become a column spanner or cease to be one even if column-span doesn't change.

Source/core/layout/LayoutMultiColumnFlowThread.h

 /*
  * 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 160 matching lines @@
     virtual LayoutPoint visualPointToFlowThreadPoint(const LayoutPoint& visualPoint) const override;
 
     virtual LayoutMultiColumnSet* columnSetAtBlockOffset(LayoutUnit) const override final;
 
     void layoutColumns(bool relayoutChildren, SubtreeLayoutScope&);
 
     bool recalculateColumnHeights();
 
     void columnRuleStyleDidChange();
 
+    // Remove the spanner placeholder and return true if the specified object is no longer a valid spanner.
+    bool removeSpannerPlaceholderIfNoLongerValid(LayoutBox* spannerObjectInFlowThread);
+
     virtual const char* name() const override { return "LayoutMultiColumnFlowThread"; }
 
 protected:
     LayoutMultiColumnFlowThread();
     void setProgressionIsInline(bool isInline) { m_progressionIsInline = isInline; }
 
     virtual void layout() override;
 
 private:
     void calculateColumnCountAndWidth(LayoutUnit& width, unsigned& count) const;
     void createAndInsertMultiColumnSet(LayoutBox* insertBefore = nullptr);
     void createAndInsertSpannerPlaceholder(LayoutBox* spannerObjectInFlowThread, LayoutObject* insertedBeforeInFlowThread);
     void destroySpannerPlaceholder(LayoutMultiColumnSpannerPlaceholder*);
     virtual bool descendantIsValidColumnSpanner(LayoutObject* descendant) const;
 
     virtual void addColumnSetToThread(LayoutMultiColumnSet*) override;
     virtual void willBeRemovedFromTree() override;
     virtual LayoutUnit skipColumnSpanner(LayoutBox*, LayoutUnit logicalTopInFlowThread) override;
-    virtual void flowThreadDescendantWasInserted(LayoutObject*) override;
-    virtual void flowThreadDescendantWillBeRemoved(LayoutObject*) override;
+    virtual void flowThreadDescendantWasInserted(LayoutObject*) final;
+    virtual void flowThreadDescendantWillBeRemoved(LayoutObject*) final;
     virtual void flowThreadDescendantStyleWillChange(LayoutObject*, StyleDifference, const ComputedStyle& newStyle) override;
     virtual void flowThreadDescendantStyleDidChange(LayoutObject*, StyleDifference, const ComputedStyle& oldStyle) override;
     virtual void computePreferredLogicalWidths() override;
     virtual void computeLogicalHeight(LayoutUnit logicalHeight, LayoutUnit logicalTop, LogicalExtentComputedValues&) const override;
     virtual void updateLogicalWidth() override;
     virtual void setPageBreak(LayoutUnit offset, LayoutUnit spaceShortage) override;
     virtual void updateMinimumPageHeight(LayoutUnit offset, LayoutUnit minHeight) override;
     virtual bool addForcedColumnBreak(LayoutUnit, LayoutObject* breakChild, bool isBefore, LayoutUnit* offsetBreakAdjustment = nullptr) override;
     virtual bool isPageLogicalHeightKnown() const override;
 
     // The last set we worked on. It's not to be used as the "current set". The concept of a
     // "current set" is difficult, since layout may jump back and forth in the tree, due to wrong
     // top location estimates (due to e.g. margin collapsing), and possibly for other reasons.
     LayoutMultiColumnSet* m_lastSetWorkedOn;
 
     unsigned m_columnCount; // The used value of column-count
     LayoutUnit m_columnHeightAvailable; // Total height available to columns, or 0 if auto.
     bool m_inBalancingPass; // Set when relayouting for column balancing.
     bool m_needsColumnHeightsRecalculation; // Set when we need to recalculate the column set heights after layout.
     bool m_progressionIsInline; // Always true for regular multicol. False for paged-y overflow.
     bool m_isBeingEvacuated;
 };
 
 } // namespace blink
 
 #endif // LayoutMultiColumnFlowThread_h