Disallow relayout roots inside multicol.

Source/core/layout/LayoutObject.cpp

Index: Source/core/layout/LayoutObject.cpp
diff --git a/Source/core/layout/LayoutObject.cpp b/Source/core/layout/LayoutObject.cpp
index 7e9ab723a1c2e308924e62d2f579e4edf9eb9d4e..50ed1fff8f775fd5b99a70006c1b2bba4162cc82 100644
--- a/Source/core/layout/LayoutObject.cpp
+++ b/Source/core/layout/LayoutObject.cpp
@@ -742,6 +742,16 @@ static inline bool objectIsRelayoutBoundary(const LayoutObject* object)
     if (object->isTablePart())
         return false;
 
+    // Inside multicol it's generally problematic to allow relayout roots. The multicol container
+    // itself may be scheduled for relayout as well (due to other changes that may have happened
+    // since the previous layout pass), which might affect the column heights, which may affect how
+    // this object breaks across columns). Spanners may also have been added or removed since the
+    // previous layout pass, which is just another way of affecting the column heights (and the
+    // number of rows). Instead of identifying cases where it's safe to allow relayout roots, just
+    // disallow them inside multicol.
+    if (object->isInsideFlowThread())
+        return false;
+
     return true;
 }