diff --git a/pcbnew/connectivity_data.h b/pcbnew/connectivity_data.h
index fb614f07a3..898542eb51 100644
--- a/pcbnew/connectivity_data.h
+++ b/pcbnew/connectivity_data.h
@@ -34,6 +34,8 @@
 #include <memory>
 
 #include <math/vector2d.h>
+#include <geometry/shape_poly_set.h>
+#include <class_zone.h>
 
 class CN_CLUSTER;
 class CN_CONNECTIVITY_ALGO;
@@ -57,8 +59,14 @@ struct CN_DISJOINT_NET_ENTRY
 
 struct CN_ZONE_ISOLATED_ISLAND_LIST
 {
-    ZONE_CONTAINER *m_zone;
-    std::vector<int> m_islands;
+    CN_ZONE_ISOLATED_ISLAND_LIST( ZONE_CONTAINER* aZone ) :
+        m_zone( aZone ),
+        m_lastPolys( aZone->GetFilledPolysList() )
+    {}
+
+    ZONE_CONTAINER*      m_zone;
+    const SHAPE_POLY_SET m_lastPolys;
+    std::vector<int>     m_islands;
 };
 
 struct RN_DYNAMIC_LINE
diff --git a/pcbnew/zone_filler.cpp b/pcbnew/zone_filler.cpp
index dec08fe38f..4b8577975c 100644
--- a/pcbnew/zone_filler.cpp
+++ b/pcbnew/zone_filler.cpp
@@ -97,9 +97,7 @@ void ZONE_FILLER::Fill( std::vector<ZONE_CONTAINER*> aZones, bool aCheck )
         if( zone->GetIsKeepout() )
             continue;
 
-        CN_ZONE_ISOLATED_ISLAND_LIST l;
-        l.m_zone = zone;
-        toFill.push_back( l );
+        toFill.emplace_back( CN_ZONE_ISOLATED_ISLAND_LIST(zone) );
     }
 
     for( unsigned i = 0; i < toFill.size(); i++ )
@@ -117,7 +115,6 @@ void ZONE_FILLER::Fill( std::vector<ZONE_CONTAINER*> aZones, bool aCheck )
     }
 
     m_next = 0;
-    m_out_of_date = false;
     m_count_done = 0;
     std::vector<std::thread> fillWorkers;
 
@@ -132,9 +129,6 @@ void ZONE_FILLER::Fill( std::vector<ZONE_CONTAINER*> aZones, bool aCheck )
                 ZONE_CONTAINER* zone = toFill[i].m_zone;
                 fillSingleZone( zone, rawPolys, finalPolys );
 
-                if( aCheck && zone->GetFilledPolysList().GetHash() != finalPolys.GetHash() )
-                    m_out_of_date.store( true );
-
                 zone->SetRawPolysList( rawPolys );
                 zone->SetFilledPolysList( finalPolys );
                 zone->SetIsFilled( true );
@@ -170,6 +164,8 @@ void ZONE_FILLER::Fill( std::vector<ZONE_CONTAINER*> aZones, bool aCheck )
     connectivity->SetProgressReporter( m_progressReporter );
     connectivity->FindIsolatedCopperIslands( toFill );
 
+    bool outOfDate = false;
+
     for( auto& zone : toFill )
     {
         std::sort( zone.m_islands.begin(), zone.m_islands.end(), std::greater<int>() );
@@ -181,11 +177,14 @@ void ZONE_FILLER::Fill( std::vector<ZONE_CONTAINER*> aZones, bool aCheck )
         }
 
         zone.m_zone->SetFilledPolysList( poly );
+
+        if( aCheck && zone.m_lastPolys.GetHash() != poly.GetHash() )
+            outOfDate = true;
     }
 
-    if( aCheck && m_out_of_date )
+    if( aCheck && outOfDate )
     {
-        bool cancel = !IsOK( nullptr, _( "Zone fills may be out-of-date. Re-fill all zones?" ) );
+        bool cancel = !IsOK( nullptr, _( "Zone fills are out-of-date. Re-fill?" ) );
 
         if( m_progressReporter )
         {
diff --git a/pcbnew/zone_filler.h b/pcbnew/zone_filler.h
index a166d1d714..b505f6ea24 100644
--- a/pcbnew/zone_filler.h
+++ b/pcbnew/zone_filler.h
@@ -122,7 +122,6 @@ private:
     std::atomic_size_t m_next;          // An index into the vector of zones to fill.
                                         // Used by the variuos parallel thread sets during
                                         // fill operations.
-    std::atomic_bool   m_out_of_date;
     std::atomic_size_t m_count_done;
 };