Defer monitor tile update when placed by another TE

src/main/java/dan200/computercraft/shared/peripheral/monitor/BlockMonitor.java

@@ -20,6 +20,7 @@ import net.minecraft.tileentity.TileEntityType;
 import net.minecraft.util.Direction;
 import net.minecraft.util.math.BlockPos;
 import net.minecraft.world.World;
+import net.minecraftforge.common.util.FakePlayer;
 import net.minecraftforge.fml.RegistryObject;
 
 import javax.annotation.Nonnull;
@@ -85,9 +86,14 @@ public class BlockMonitor extends BlockGeneric
         if( entity instanceof TileMonitor && !world.isClientSide )
         {
             TileMonitor monitor = (TileMonitor) entity;
-            monitor.contractNeighbours();
+            // Defer the block update if we're being placed by another TE. See #691
-            monitor.contract();
+            if ( livingEntity == null || livingEntity instanceof FakePlayer )
-            monitor.expand();
+            {
+                monitor.updateNeighborsDeferred();
+                return;
+            }
+
+            monitor.updateNeighbors();
         }
     }
 }

src/main/java/dan200/computercraft/shared/peripheral/monitor/TileMonitor.java

@@ -55,6 +55,7 @@ public class TileMonitor extends TileGeneric
     private LazyOptional<IPeripheral> peripheralCap;
     private final Set<IComputerAccess> computers = new HashSet<>();
 
+    private boolean needsUpdate = false;
     private boolean destroyed = false;
     private boolean visiting = false;
 
@@ -147,6 +148,12 @@ public class TileMonitor extends TileGeneric
     @Override
     public void blockTick()
     {
+        if ( needsUpdate )
+        {
+            needsUpdate = false;
+            updateNeighbors();
+        }
+
         if( xIndex != 0 || yIndex != 0 || serverMonitor == null ) return;
 
         serverMonitor.clearChanged();
@@ -525,6 +532,18 @@ public class TileMonitor extends TileGeneric
         return true;
     }
 
+    void updateNeighborsDeferred()
+    {
+        needsUpdate = true;
+    }
+
+    void updateNeighbors()
+    {
+        contractNeighbours();
+        contract();
+        expand();
+    }
+
     @SuppressWarnings( "StatementWithEmptyBody" )
     void expand()
     {