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

Source/core/layout/LayoutMultiColumnSpannerPlaceholder.h

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef LayoutMultiColumnSpannerPlaceholder_h
 #define LayoutMultiColumnSpannerPlaceholder_h
 
 #include "core/layout/LayoutMultiColumnFlowThread.h"
 
 namespace blink {
 
 // Placeholder layoutObject for column-span:all elements. The column-span:all layoutObject itself is a
 // descendant of the flow thread, but due to its out-of-flow nature, we need something on the
 // outside to take care of its positioning and sizing. LayoutMultiColumnSpannerPlaceholder objects
 // are siblings of LayoutMultiColumnSet objects, i.e. direct children of the multicol container.
 class LayoutMultiColumnSpannerPlaceholder final : public LayoutBox {
 public:
     virtual bool isOfType(LayoutObjectType type) const override { return type == LayoutObjectLayoutMultiColumnSpannerPlaceholder || LayoutBox::isOfType(type); }
 
     static LayoutMultiColumnSpannerPlaceholder* createAnonymous(const ComputedStyle& parentStyle, LayoutBox&);
 
-    LayoutFlowThread* flowThread() const { return toLayoutBlockFlow(parent())->multiColumnFlowThread(); }
+    LayoutMultiColumnFlowThread* flowThread() const { return toLayoutBlockFlow(parent())->multiColumnFlowThread(); }
 
     LayoutBox* layoutObjectInFlowThread() const { return m_layoutObjectInFlowThread; }
     void markForLayoutIfObjectInFlowThreadNeedsLayout()
     {
         if (!m_layoutObjectInFlowThread->needsLayout())
             return;
         // The containing block of a spanner is the multicol container (our parent here), but the
         // spanner is laid out via its spanner set (us), so we need to make sure that we enter it.
         setChildNeedsLayout(MarkOnlyThis);
     }
+
+    void layoutObjectInFlowThreadStyleDidChange(const ComputedStyle* oldStyle);
     void updateMarginProperties();
 
     virtual const char* name() const override { return "LayoutMultiColumnSpannerPlaceholder"; }
 
 protected:
     virtual void willBeRemovedFromTree() override;
     virtual bool needsPreferredWidthsRecalculation() const override;
     virtual LayoutUnit minPreferredLogicalWidth() const override;
     virtual LayoutUnit maxPreferredLogicalWidth() const override;
     virtual void layout() override;
     virtual void computeLogicalHeight(LayoutUnit logicalHeight, LayoutUnit logicalTop, LogicalExtentComputedValues&) const override;
     virtual void invalidateTreeIfNeeded(PaintInvalidationState&) override;
     virtual void paint(const PaintInfo&, const LayoutPoint& paintOffset) override;
     virtual bool nodeAtPoint(HitTestResult&, const HitTestLocation& locationInContainer, const LayoutPoint& accumulatedOffset, HitTestAction) override;
 
 private:
     LayoutMultiColumnSpannerPlaceholder(LayoutBox*);
 
     LayoutBox* m_layoutObjectInFlowThread; // The actual column-span:all layoutObject inside the flow thread.
 };
 
 DEFINE_LAYOUT_OBJECT_TYPE_CASTS(LayoutMultiColumnSpannerPlaceholder, isLayoutMultiColumnSpannerPlaceholder());
 
 } // namespace blink
 
 #endif