diff --git a/pcbnew/board.cpp b/pcbnew/board.cpp
index 96e15c1cd2..964a7fc408 100644
--- a/pcbnew/board.cpp
+++ b/pcbnew/board.cpp
@@ -263,6 +263,7 @@ void BOARD::IncrementTimeStamp()
         m_DRCMaxPhysicalClearance = 0;
         m_DRCZones.clear();
         m_DRCCopperZones.clear();
+        m_ZoneIsolatedIslandsMap.clear();
         m_CopperZoneRTreeCache.clear();
         m_CopperItemRTreeCache = std::make_unique<DRC_RTREE>();
     }
diff --git a/pcbnew/board.h b/pcbnew/board.h
index 2d64f8cbac..d727d5489d 100644
--- a/pcbnew/board.h
+++ b/pcbnew/board.h
@@ -60,6 +60,7 @@ class CONNECTIVITY_DATA;
 class COMPONENT;
 class PROJECT;
 class PROGRESS_REPORTER;
+struct ISOLATED_ISLANDS;
 
 // The default value for m_outlinesChainingEpsilon to convert a board outlines to polygons
 // It is the max dist between 2 end points to see them connected
@@ -1177,6 +1178,7 @@ public:
     int                   m_DRCMaxClearance;
     int                   m_DRCMaxPhysicalClearance;
     ZONE*                 m_SolderMask;
+    std::map<ZONE*, std::map<PCB_LAYER_ID, ISOLATED_ISLANDS>> m_ZoneIsolatedIslandsMap;
 
 private:
     // The default copy constructor & operator= are inadequate,
diff --git a/pcbnew/connectivity/connectivity_algo.cpp b/pcbnew/connectivity/connectivity_algo.cpp
index 49cfab806e..528ce9d916 100644
--- a/pcbnew/connectivity/connectivity_algo.cpp
+++ b/pcbnew/connectivity/connectivity_algo.cpp
@@ -613,53 +613,26 @@ void CN_CONNECTIVITY_ALGO::PropagateNets( BOARD_COMMIT* aCommit )
 }
 
 
-void CN_CONNECTIVITY_ALGO::FindIsolatedCopperIslands( ZONE* aZone, PCB_LAYER_ID aLayer,
-                                                      std::vector<int>& aIslands )
-{
-    if( aZone->GetFilledPolysList( aLayer )->IsEmpty() )
-        return;
-
-    aIslands.clear();
-
-    Remove( aZone );
-    Add( aZone );
-
-    m_connClusters = SearchClusters( CSM_CONNECTIVITY_CHECK );
-
-    for( const std::shared_ptr<CN_CLUSTER>& cluster : m_connClusters )
-    {
-        if( cluster->Contains( aZone ) && cluster->IsOrphaned() )
-        {
-            for( CN_ITEM* z : *cluster )
-            {
-                if( z->Parent() == aZone && z->Layer() == aLayer )
-                    aIslands.push_back( static_cast<CN_ZONE_LAYER*>(z)->SubpolyIndex() );
-            }
-        }
-    }
-
-    wxLogTrace( wxT( "CN" ), wxT( "Found %u isolated islands\n" ), (unsigned) aIslands.size() );
-}
-
-void CN_CONNECTIVITY_ALGO::FindIsolatedCopperIslands( std::vector<CN_ZONE_ISOLATED_ISLAND_LIST>& aZones,
-                                                      bool aConnectivityAlreadyRebuilt )
+void CN_CONNECTIVITY_ALGO::FillIsolatedIslandsMap(
+                                std::map<ZONE*, std::map<PCB_LAYER_ID, ISOLATED_ISLANDS>>& aMap,
+                                bool aConnectivityAlreadyRebuilt )
 {
     int progressDelta = 50;
     int ii = 0;
 
-    progressDelta = std::max( progressDelta, (int) aZones.size() / 4 );
+    progressDelta = std::max( progressDelta, (int) aMap.size() / 4 );
 
     if( !aConnectivityAlreadyRebuilt )
     {
-        for( CN_ZONE_ISOLATED_ISLAND_LIST& z : aZones )
+        for( const auto& [ zone, islands ] : aMap )
         {
-            Remove( z.m_zone );
-            Add( z.m_zone );
+            Remove( zone );
+            Add( zone );
             ii++;
 
             if( m_progressReporter && ( ii % progressDelta ) == 0 )
             {
-                m_progressReporter->SetCurrentProgress( (double) ii / (double) aZones.size() );
+                m_progressReporter->SetCurrentProgress( (double) ii / (double) aMap.size() );
                 m_progressReporter->KeepRefreshing( false );
             }
 
@@ -670,24 +643,25 @@ void CN_CONNECTIVITY_ALGO::FindIsolatedCopperIslands( std::vector<CN_ZONE_ISOLAT
 
     m_connClusters = SearchClusters( CSM_CONNECTIVITY_CHECK );
 
-    for( CN_ZONE_ISOLATED_ISLAND_LIST& zone : aZones )
+    for( auto& [ zone, zoneIslands ] : aMap )
     {
-        for( PCB_LAYER_ID layer : zone.m_zone->GetLayerSet().Seq() )
+        for( auto& [ layer, layerIslands ] : zoneIslands )
         {
-            if( zone.m_zone->GetFilledPolysList( layer )->IsEmpty() )
+            if( zone->GetFilledPolysList( layer )->IsEmpty() )
                 continue;
 
             for( const std::shared_ptr<CN_CLUSTER>& cluster : m_connClusters )
             {
-                if( cluster->Contains( zone.m_zone ) && cluster->IsOrphaned() )
+                for( CN_ITEM* item : *cluster )
                 {
-                    for( CN_ITEM* z : *cluster )
+                    if( item->Parent() == zone && item->Layer() == layer )
                     {
-                        if( z->Parent() == zone.m_zone && z->Layer() == layer )
-                        {
-                            zone.m_islands[layer].push_back(
-                                    static_cast<CN_ZONE_LAYER*>( z )->SubpolyIndex() );
-                        }
+                        CN_ZONE_LAYER* z = static_cast<CN_ZONE_LAYER*>( item );
+
+                        if( cluster->IsOrphaned() )
+                            layerIslands.m_IsolatedOutlines.push_back( z->SubpolyIndex() );
+                        else if( z->HasSingleConnection() )
+                            layerIslands.m_SingleConnectionOutlines.push_back( z->SubpolyIndex() );
                     }
                 }
             }
diff --git a/pcbnew/connectivity/connectivity_algo.h b/pcbnew/connectivity/connectivity_algo.h
index f903a98ad8..14ace5a782 100644
--- a/pcbnew/connectivity/connectivity_algo.h
+++ b/pcbnew/connectivity/connectivity_algo.h
@@ -221,18 +221,11 @@ public:
      */
     void PropagateNets( BOARD_COMMIT* aCommit = nullptr );
 
-    void FindIsolatedCopperIslands( ZONE* aZone, PCB_LAYER_ID aLayer, std::vector<int>& aIslands );
-
     /**
-     * Find the copper islands that are not connected to a net.
-     *
-     * These are added to the m_islands vector.
-     * N.B. This must be called after aZones has been refreshed.
-     *
-     * @param: aZones is the set of zones to search for islands.
+     * Fill in the isolated islands map with copper islands that are not connected to a net.
      */
-    void FindIsolatedCopperIslands( std::vector<CN_ZONE_ISOLATED_ISLAND_LIST>& aZones,
-                                    bool aConnectivityAlreadyRebuilt );
+    void FillIsolatedIslandsMap( std::map<ZONE*, std::map<PCB_LAYER_ID, ISOLATED_ISLANDS>>& aMap,
+                                 bool aConnectivityAlreadyRebuilt );
 
     const CLUSTERS& GetClusters();
 
diff --git a/pcbnew/connectivity/connectivity_data.cpp b/pcbnew/connectivity/connectivity_data.cpp
index 3f4ad79247..799c4b8ffd 100644
--- a/pcbnew/connectivity/connectivity_data.cpp
+++ b/pcbnew/connectivity/connectivity_data.cpp
@@ -309,18 +309,10 @@ int CONNECTIVITY_DATA::GetNetCount() const
 }
 
 
-void CONNECTIVITY_DATA::FindIsolatedCopperIslands( ZONE* aZone, std::vector<int>& aIslands )
+void CONNECTIVITY_DATA::FillIsolatedIslandsMap( std::map<ZONE*, std::map<PCB_LAYER_ID, ISOLATED_ISLANDS>>& aMap,
+                                                bool aConnectivityAlreadyRebuilt )
 {
-    // TODO(JE) ZONES
-#if 0
-    m_connAlgo->FindIsolatedCopperIslands( aZone, aIslands );
-#endif
-}
-
-void CONNECTIVITY_DATA::FindIsolatedCopperIslands( std::vector<CN_ZONE_ISOLATED_ISLAND_LIST>& aZones,
-                                                   bool aConnectivityAlreadyRebuilt )
-{
-    m_connAlgo->FindIsolatedCopperIslands( aZones, aConnectivityAlreadyRebuilt );
+    m_connAlgo->FillIsolatedIslandsMap( aMap, aConnectivityAlreadyRebuilt );
 }
 
 
diff --git a/pcbnew/connectivity/connectivity_data.h b/pcbnew/connectivity/connectivity_data.h
index 97f6397c92..85e0ea742b 100644
--- a/pcbnew/connectivity/connectivity_data.h
+++ b/pcbnew/connectivity/connectivity_data.h
@@ -64,21 +64,6 @@ struct CN_DISJOINT_NET_ENTRY
 };
 
 
-/**
- * A structure used for calculating isolated islands on a given zone across all its layers
- */
-struct CN_ZONE_ISOLATED_ISLAND_LIST
-{
-    CN_ZONE_ISOLATED_ISLAND_LIST( ZONE* aZone ) :
-            m_zone( aZone )
-    {}
-
-    ZONE* m_zone;
-
-    std::map<PCB_LAYER_ID, std::vector<int>> m_islands;
-};
-
-
 struct RN_DYNAMIC_LINE
 {
     int netCode;
@@ -176,14 +161,11 @@ public:
     void PropagateNets( BOARD_COMMIT* aCommit = nullptr );
 
     /**
-     * Function FindIsolatedCopperIslands()
-     * Searches for copper islands in zone aZone that are not connected to any pad.
-     * @param aZone zone to test
-     * @param aIslands list of islands that have no connections (outline indices in the polygon set)
+     * Fill the isolate islands list for each layer of each zone.  Isolated islands are individual
+     * polygons in a zone fill that don't connect to a net.
      */
-    void FindIsolatedCopperIslands( ZONE* aZone, std::vector<int>& aIslands );
-    void FindIsolatedCopperIslands( std::vector<CN_ZONE_ISOLATED_ISLAND_LIST>& aZones,
-                                    bool aConnectivityAlreadyRebuilt = false );
+    void FillIsolatedIslandsMap( std::map<ZONE*, std::map<PCB_LAYER_ID, ISOLATED_ISLANDS>>& aMap,
+                                 bool aConnectivityAlreadyRebuilt = false );
 
     /**
      * Function RecalculateRatsnest()
diff --git a/pcbnew/connectivity/connectivity_items.cpp b/pcbnew/connectivity/connectivity_items.cpp
index 1803fb928e..da635f2ca5 100644
--- a/pcbnew/connectivity/connectivity_items.cpp
+++ b/pcbnew/connectivity/connectivity_items.cpp
@@ -109,6 +109,23 @@ const VECTOR2I CN_ZONE_LAYER::GetAnchor( int n ) const
 }
 
 
+bool CN_ZONE_LAYER::HasSingleConnection()
+{
+    int count = 0;
+
+    for( CN_ITEM* item : ConnectedItems() )
+    {
+        if( item->Valid() )
+            count++;
+
+        if( count > 1 )
+            break;
+    }
+
+    return count == 1;
+}
+
+
 void CN_ITEM::RemoveInvalidRefs()
 {
     for( auto it = m_connected.begin(); it != m_connected.end(); /* increment in loop */ )
diff --git a/pcbnew/connectivity/connectivity_items.h b/pcbnew/connectivity/connectivity_items.h
index b52b33b735..36a4925de0 100644
--- a/pcbnew/connectivity/connectivity_items.h
+++ b/pcbnew/connectivity/connectivity_items.h
@@ -387,6 +387,8 @@ public:
         return collision;
     }
 
+    bool HasSingleConnection();
+
 private:
     int                                 m_subpolyIndex;
     PCB_LAYER_ID                        m_layer;
diff --git a/pcbnew/drc/drc_cache_generator.cpp b/pcbnew/drc/drc_cache_generator.cpp
index 652ca5d265..c0f903dd03 100644
--- a/pcbnew/drc/drc_cache_generator.cpp
+++ b/pcbnew/drc/drc_cache_generator.cpp
@@ -26,7 +26,7 @@
 #include <footprint.h>
 #include <thread_pool.h>
 #include <zone.h>
-
+#include <connectivity/connectivity_data.h>
 #include <drc/drc_engine.h>
 #include <drc/drc_rtree.h>
 #include <drc/drc_cache_generator.h>
@@ -201,6 +201,23 @@ bool DRC_CACHE_GENERATOR::Run()
         }
     }
 
+    m_board->m_ZoneIsolatedIslandsMap.clear();
+
+    for( ZONE* zone : m_board->Zones() )
+    {
+        if( !zone->GetIsRuleArea() )
+        {
+            for( PCB_LAYER_ID layer : zone->GetLayerSet().Seq() )
+                m_board->m_ZoneIsolatedIslandsMap[ zone ][ layer ] = ISOLATED_ISLANDS();
+        }
+    }
+
+    std::shared_ptr<CONNECTIVITY_DATA> connectivity = m_board->GetConnectivity();
+
+    connectivity->ClearRatsnest();
+    connectivity->Build( m_board, m_drcEngine->GetProgressReporter() );
+    connectivity->FillIsolatedIslandsMap( m_board->m_ZoneIsolatedIslandsMap, true );
+
     return !m_drcEngine->IsCancelled();
 }
 
diff --git a/pcbnew/drc/drc_test_provider_connectivity.cpp b/pcbnew/drc/drc_test_provider_connectivity.cpp
index 562c513337..111f512478 100644
--- a/pcbnew/drc/drc_test_provider_connectivity.cpp
+++ b/pcbnew/drc/drc_test_provider_connectivity.cpp
@@ -26,7 +26,7 @@
 
 #include <connectivity/connectivity_data.h>
 #include <connectivity/connectivity_algo.h>
-
+#include <zone.h>
 #include <drc/drc_engine.h>
 #include <drc/drc_item.h>
 #include <drc/drc_rule.h>
@@ -71,25 +71,12 @@ bool DRC_TEST_PROVIDER_CONNECTIVITY::Run()
     if( !reportPhase( _( "Checking pad, via and zone connections..." ) ) )
         return false;   // DRC cancelled
 
-    BOARD* board = m_drcEngine->GetBoard();
-
-    std::shared_ptr<CONNECTIVITY_DATA>        connectivity = board->GetConnectivity();
-    std::vector<CN_ZONE_ISOLATED_ISLAND_LIST> islandsList;
-
-    for( ZONE* zone : board->Zones() )
-    {
-        if( !zone->GetIsRuleArea() )
-            islandsList.emplace_back( CN_ZONE_ISOLATED_ISLAND_LIST( zone ) );
-    }
-
-    // Rebuild (from scratch, ignoring dirty flags) just in case. This really needs to be reliable.
-    connectivity->ClearRatsnest();
-    connectivity->Build( board, m_drcEngine->GetProgressReporter() );
-    connectivity->FindIsolatedCopperIslands( islandsList, true );
+    BOARD*                             board = m_drcEngine->GetBoard();
+    std::shared_ptr<CONNECTIVITY_DATA> connectivity = board->GetConnectivity();
 
     int progressDelta = 250;
     int ii = 0;
-    int count = board->Tracks().size() + islandsList.size();
+    int count = board->Tracks().size() + board->m_ZoneIsolatedIslandsMap.size();
 
     ii += count;      // We gave half of this phase to CONNECTIVITY_DATA::Build()
     count += count;
@@ -122,7 +109,7 @@ bool DRC_TEST_PROVIDER_CONNECTIVITY::Run()
     }
 
     /* test starved zones */
-    for( CN_ZONE_ISOLATED_ISLAND_LIST& zone : islandsList )
+    for( const auto& [ zone, zoneIslands ] : board->m_ZoneIsolatedIslandsMap )
     {
         if( m_drcEngine->IsErrorLimitExceeded( DRCE_ISOLATED_COPPER ) )
             break;
@@ -130,21 +117,18 @@ bool DRC_TEST_PROVIDER_CONNECTIVITY::Run()
         if( !reportProgress( ii++, count, progressDelta ) )
             return false;   // DRC cancelled
 
-        for( PCB_LAYER_ID layer : zone.m_zone->GetLayerSet().Seq() )
+        for( const auto& [ layer, layerIslands ] : zoneIslands )
         {
-            if( !zone.m_islands.count( layer ) )
-                continue;
-
-            std::shared_ptr<SHAPE_POLY_SET> poly = zone.m_zone->GetFilledPolysList( layer );
-
-            for( int idx : zone.m_islands.at( layer ) )
+            for( int polyIdx : layerIslands.m_IsolatedOutlines )
             {
                 if( m_drcEngine->IsErrorLimitExceeded( DRCE_ISOLATED_COPPER ) )
                     break;
 
+                std::shared_ptr<SHAPE_POLY_SET> poly = zone->GetFilledPolysList( layer );
+
                 std::shared_ptr<DRC_ITEM> drcItem = DRC_ITEM::Create( DRCE_ISOLATED_COPPER );
-                drcItem->SetItems( zone.m_zone );
-                reportViolation( drcItem, poly->Outline( idx ).CPoint( 0 ), layer );
+                drcItem->SetItems( zone );
+                reportViolation( drcItem, poly->Outline( polyIdx ).CPoint( 0 ), layer );
             }
         }
     }
diff --git a/pcbnew/drc/drc_test_provider_zone_connections.cpp b/pcbnew/drc/drc_test_provider_zone_connections.cpp
index e90c216b4b..737e02e159 100644
--- a/pcbnew/drc/drc_test_provider_zone_connections.cpp
+++ b/pcbnew/drc/drc_test_provider_zone_connections.cpp
@@ -81,6 +81,17 @@ void DRC_TEST_PROVIDER_ZONE_CONNECTIONS::testZoneLayer( ZONE* aZone, PCB_LAYER_I
     DRC_CONSTRAINT                     constraint;
 
     const std::shared_ptr<SHAPE_POLY_SET>& zoneFill = aZone->GetFilledPolysList( aLayer );
+    ISOLATED_ISLANDS                       isolatedIslands;
+
+    auto zoneIter = board->m_ZoneIsolatedIslandsMap.find( aZone );
+
+    if( zoneIter != board->m_ZoneIsolatedIslandsMap.end() )
+    {
+        auto layerIter = zoneIter->second.find( aLayer );
+
+        if( layerIter != zoneIter->second.end() )
+            isolatedIslands = layerIter->second;
+    }
 
     for( FOOTPRINT* footprint : board->Footprints() )
     {
@@ -132,7 +143,15 @@ void DRC_TEST_PROVIDER_ZONE_CONNECTIONS::testZoneLayer( ZONE* aZone, PCB_LAYER_I
             std::vector<SHAPE_LINE_CHAIN::INTERSECTION> intersections;
 
             for( int jj = 0; jj < zoneFill->OutlineCount(); ++jj )
+            {
+                // If we connect to an island that only connects to a single item then we *are*
+                // that item.  Thermal spokes to this (otherwise isolated) island don't provide
+                // electrical connectivity to anything, so we don't count them.
+                if( alg::contains( isolatedIslands.m_SingleConnectionOutlines, jj ) )
+                    continue;
+
                 zoneFill->Outline( jj ).Intersect( padOutline, intersections, true, &padBBox );
+            }
 
             int spokes = intersections.size() / 2;
 
@@ -198,10 +217,13 @@ bool DRC_TEST_PROVIDER_ZONE_CONNECTIONS::Run()
 
     for( ZONE* zone : board->m_DRCCopperZones )
     {
-        for( PCB_LAYER_ID layer : zone->GetLayerSet().Seq() )
+        if( !zone->IsTeardropArea() )
         {
-            zoneLayers.push_back( { zone, layer } );
-            total_effort += zone->GetFilledPolysList( layer )->FullPointCount();
+            for( PCB_LAYER_ID layer : zone->GetLayerSet().Seq() )
+            {
+                zoneLayers.push_back( { zone, layer } );
+                total_effort += zone->GetFilledPolysList( layer )->FullPointCount();
+            }
         }
     }
 
diff --git a/pcbnew/zone.h b/pcbnew/zone.h
index 4330561ba3..50794d491c 100644
--- a/pcbnew/zone.h
+++ b/pcbnew/zone.h
@@ -44,6 +44,24 @@ class ZONE;
 class MSG_PANEL_ITEM;
 
 
+/**
+ * A struct recording the isolated and single-pad islands within a zone.  Each array holds
+ * indexes into the outlines of a SHAPE_POLY_SET for a zone fill on a particular layer.
+ *
+ * Isolated outlines are those whose *connectivity cluster* contains no pads.  These generate
+ * DRC violations.
+ *
+ * Single-connection outlines are those with a *direct* connection to only a single item.  These
+ * participate in thermal spoke counting as a pad spoke to an *otherwise* unconnected island
+ * provides no connectivity to the pad.
+ */
+struct ISOLATED_ISLANDS
+{
+    std::vector<int> m_IsolatedOutlines;
+    std::vector<int> m_SingleConnectionOutlines;
+};
+
+
 /**
  * Handle a list of polygons defining a copper zone.
  *
diff --git a/pcbnew/zone_filler.cpp b/pcbnew/zone_filler.cpp
index 9821bc4dc3..004db46045 100644
--- a/pcbnew/zone_filler.cpp
+++ b/pcbnew/zone_filler.cpp
@@ -90,9 +90,9 @@ bool ZONE_FILLER::Fill( std::vector<ZONE*>& aZones, bool aCheck, wxWindow* aPare
 {
     std::lock_guard<KISPINLOCK> lock( m_board->GetConnectivity()->GetLock() );
 
-    std::vector<std::pair<ZONE*, PCB_LAYER_ID>>        toFill;
-    std::map<std::pair<ZONE*, PCB_LAYER_ID>, MD5_HASH> oldFillHashes;
-    std::vector<CN_ZONE_ISOLATED_ISLAND_LIST>          islandsList;
+    std::vector<std::pair<ZONE*, PCB_LAYER_ID>>               toFill;
+    std::map<std::pair<ZONE*, PCB_LAYER_ID>, MD5_HASH>        oldFillHashes;
+    std::map<ZONE*, std::map<PCB_LAYER_ID, ISOLATED_ISLANDS>> isolatedIslandsMap;
 
     std::shared_ptr<CONNECTIVITY_DATA> connectivity = m_board->GetConnectivity();
 
@@ -328,9 +328,9 @@ bool ZONE_FILLER::Fill( std::vector<ZONE*>& aZones, bool aCheck, wxWindow* aPare
 
             // Add the zone to the list of zones to test or refill
             toFill.emplace_back( std::make_pair( zone, layer ) );
-        }
 
-        islandsList.emplace_back( CN_ZONE_ISOLATED_ISLAND_LIST( zone ) );
+            isolatedIslandsMap[ zone ][ layer ] = ISOLATED_ISLANDS();
+        }
 
         // Remove existing fill first to prevent drawing invalid polygons on some platforms
         zone->UnFill();
@@ -534,7 +534,7 @@ bool ZONE_FILLER::Fill( std::vector<ZONE*>& aZones, bool aCheck, wxWindow* aPare
     }
 
     connectivity->SetProgressReporter( m_progressReporter );
-    connectivity->FindIsolatedCopperIslands( islandsList );
+    connectivity->FillIsolatedIslandsMap( isolatedIslandsMap );
     connectivity->SetProgressReporter( nullptr );
 
     if( m_progressReporter && m_progressReporter->IsCancelled() )
@@ -552,26 +552,15 @@ bool ZONE_FILLER::Fill( std::vector<ZONE*>& aZones, bool aCheck, wxWindow* aPare
     // Now remove isolated copper islands according to the isolated islands strategy assigned
     // by the user (always, never, below-certain-size).
     //
-    for( CN_ZONE_ISOLATED_ISLAND_LIST& zone : islandsList )
+    for( const auto& [ zone, zoneIslands ] : isolatedIslandsMap )
     {
         // If *all* the polygons are islands, do not remove any of them
         bool allIslands = true;
 
-        for( PCB_LAYER_ID layer : zone.m_zone->GetLayerSet().Seq() )
+        for( const auto& [ layer, layerIslands ] : zoneIslands )
         {
-            std::shared_ptr<SHAPE_POLY_SET> poly = zone.m_zone->GetFilledPolysList( layer );
-
-            if( !zone.m_islands.count( layer ) )
-            {
-                if( poly->OutlineCount() > 0 )
-                    allIslands = false;
-
-                continue;
-            }
-
-            std::vector<int>& islands = zone.m_islands.at( layer );
-
-            if( islands.size() != static_cast<size_t>( poly->OutlineCount() ) )
+            if( layerIslands.m_IsolatedOutlines.size()
+                    != static_cast<size_t>( zone->GetFilledPolysList( layer )->OutlineCount() ) )
             {
                 allIslands = false;
                 break;
@@ -581,23 +570,23 @@ bool ZONE_FILLER::Fill( std::vector<ZONE*>& aZones, bool aCheck, wxWindow* aPare
         if( allIslands )
             continue;
 
-        for( PCB_LAYER_ID layer : zone.m_zone->GetLayerSet().Seq() )
+        for( const auto& [ layer, layerIslands ] : zoneIslands )
         {
             if( m_debugZoneFiller && LSET::InternalCuMask().Contains( layer ) )
                 continue;
 
-            if( !zone.m_islands.count( layer ) )
+            if( layerIslands.m_IsolatedOutlines.empty() )
                 continue;
 
-            std::vector<int>& islands = zone.m_islands.at( layer );
+            std::vector<int> islands = layerIslands.m_IsolatedOutlines;
 
             // The list of polygons to delete must be explored from last to first in list,
             // to allow deleting a polygon from list without breaking the remaining of the list
             std::sort( islands.begin(), islands.end(), std::greater<int>() );
 
-            std::shared_ptr<SHAPE_POLY_SET> poly = zone.m_zone->GetFilledPolysList( layer );
-            long long int                   minArea = zone.m_zone->GetMinIslandArea();
-            ISLAND_REMOVAL_MODE             mode = zone.m_zone->GetIslandRemovalMode();
+            std::shared_ptr<SHAPE_POLY_SET> poly = zone->GetFilledPolysList( layer );
+            long long int                   minArea = zone->GetMinIslandArea();
+            ISLAND_REMOVAL_MODE             mode = zone->GetIslandRemovalMode();
 
             for( int idx : islands )
             {
@@ -608,11 +597,11 @@ bool ZONE_FILLER::Fill( std::vector<ZONE*>& aZones, bool aCheck, wxWindow* aPare
                 else if ( mode == ISLAND_REMOVAL_MODE::AREA && outline.Area( true ) < minArea )
                     poly->DeletePolygonAndTriangulationData( idx, false );
                 else
-                    zone.m_zone->SetIsIsland( layer, idx );
+                    zone->SetIsIsland( layer, idx );
             }
 
             poly->UpdateTriangulationDataHash();
-            zone.m_zone->CalculateFilledArea();
+            zone->CalculateFilledArea();
 
             if( m_progressReporter && m_progressReporter->IsCancelled() )
                 return false;
@@ -646,39 +635,42 @@ bool ZONE_FILLER::Fill( std::vector<ZONE*>& aZones, bool aCheck, wxWindow* aPare
         }
     }
 
-    auto island_lambda = [&]( int aStart, int aEnd ) -> island_check_return
-        {
-            island_check_return retval;
-
-            for( int ii = aStart; ii < aEnd && !cancelled; ++ii )
+    auto island_lambda =
+            [&]( int aStart, int aEnd ) -> island_check_return
             {
-                auto [poly, minArea] = polys_to_check[ii];
+                island_check_return retval;
 
-                for( int jj = poly->OutlineCount() - 1; jj >= 0; jj-- )
+                for( int ii = aStart; ii < aEnd && !cancelled; ++ii )
                 {
-                    SHAPE_POLY_SET island;
-                    SHAPE_POLY_SET intersection;
-                    const SHAPE_LINE_CHAIN& test_poly = poly->Polygon( jj ).front();
-                    double island_area = test_poly.Area();
+                    auto [poly, minArea] = polys_to_check[ii];
 
-                    if( island_area < minArea )
-                        continue;
+                    for( int jj = poly->OutlineCount() - 1; jj >= 0; jj-- )
+                    {
+                        SHAPE_POLY_SET island;
+                        SHAPE_POLY_SET intersection;
+                        const SHAPE_LINE_CHAIN& test_poly = poly->Polygon( jj ).front();
+                        double island_area = test_poly.Area();
+
+                        if( island_area < minArea )
+                            continue;
 
 
-                    island.AddOutline( test_poly );
-                    intersection.BooleanIntersection( m_boardOutline, island, SHAPE_POLY_SET::POLYGON_MODE::PM_FAST );
+                        island.AddOutline( test_poly );
+                        intersection.BooleanIntersection( m_boardOutline, island,
+                                                          SHAPE_POLY_SET::POLYGON_MODE::PM_FAST );
 
-                    // Nominally, all of these areas should be either inside or outside the board outline.  So this test
-                    // should be able to just compare areas (if they are equal, you are inside).  But in practice,
-                    // we sometimes can have slight overlap at the edges.  So testing against half-size area is
-                    // a fail-safe
-                    if( intersection.Area() < island_area / 2.0 )
-                        retval.emplace_back( poly, jj );
+                        // Nominally, all of these areas should be either inside or outside the
+                        // board outline.  So this test should be able to just compare areas (if
+                        // they are equal, you are inside).  But in practice, we sometimes have
+                        // slight overlap at the edges, so testing against half-size area acts as
+                        // a fail-safe.
+                        if( intersection.Area() < island_area / 2.0 )
+                            retval.emplace_back( poly, jj );
+                    }
                 }
-            }
 
-            return retval;
-        };
+                return retval;
+            };
 
     auto island_returns = tp.parallelize_loop( 0, polys_to_check.size(), island_lambda );
     cancelled = false;
@@ -1575,10 +1567,14 @@ bool ZONE_FILLER::fillCopperZone( const ZONE* aZone, PCB_LAYER_ID aLayer, PCB_LA
     DUMP_POLYS_TO_COPPER_LAYER( aFillPolys, In15_Cu, wxT( "after-reinflating" ) );
 
     /* -------------------------------------------------------------------------------------
-     * Ensure additive changes (thermal stubs and particularly inflating acute corners) do not
-     * add copper outside the zone boundary or inside the clearance holes
+     * Ensure additive changes (thermal stubs and inflating acute corners) do not add copper
+     * outside the zone boundary, inside the clearance holes, or between otherwise isolated
+     * islands
      */
 
+    for( PAD* pad : thermalConnectionPads )
+        addHoleKnockout( pad, 0, clearanceHoles );
+
     aFillPolys.BooleanIntersection( aMaxExtents, SHAPE_POLY_SET::PM_FAST );
     DUMP_POLYS_TO_COPPER_LAYER( aFillPolys, In16_Cu, wxT( "after-trim-to-outline" ) );
     aFillPolys.BooleanSubtract( clearanceHoles, SHAPE_POLY_SET::PM_FAST );