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

Source/core/layout/LayoutMultiColumnSpannerPlaceholder.h

Index: Source/core/layout/LayoutMultiColumnSpannerPlaceholder.h
diff --git a/Source/core/layout/LayoutMultiColumnSpannerPlaceholder.h b/Source/core/layout/LayoutMultiColumnSpannerPlaceholder.h
index 1b48d4af8ff8ca97dbc1ceed513361b15f6aa8da..f604173d9b908e1b0af8f162159777910c6f3473 100644
--- a/Source/core/layout/LayoutMultiColumnSpannerPlaceholder.h
+++ b/Source/core/layout/LayoutMultiColumnSpannerPlaceholder.h
@@ -19,7 +19,7 @@ public:
 
     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()
@@ -30,6 +30,8 @@ public:
         // 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"; }