PaperMC/paper-server/nms-patches/net/minecraft/world/level/block/BlockWitherSkull.patch

--- a/net/minecraft/world/level/block/BlockWitherSkull.java
+++ b/net/minecraft/world/level/block/BlockWitherSkull.java
@@ -26,6 +26,11 @@
 import net.minecraft.world.level.block.state.predicate.MaterialPredicate;
 import net.minecraft.world.level.material.Material;
 
+// CraftBukkit start
+import org.bukkit.craftbukkit.util.BlockStateListPopulator;
+import org.bukkit.event.entity.CreatureSpawnEvent.SpawnReason;
+// CraftBukkit end
+
 public class BlockWitherSkull extends BlockSkull {
 
     @Nullable
@@ -49,6 +54,7 @@
     }
 
     public static void checkSpawn(World world, BlockPosition blockposition, TileEntitySkull tileentityskull) {
+        if (world.captureBlockStates) return; // CraftBukkit
         if (!world.isClientSide) {
             IBlockData iblockdata = tileentityskull.getBlockState();
             boolean flag = iblockdata.is(Blocks.WITHER_SKELETON_SKULL) || iblockdata.is(Blocks.WITHER_SKELETON_WALL_SKULL);
@@ -58,12 +64,14 @@
                 ShapeDetector.ShapeDetectorCollection shapedetector_shapedetectorcollection = shapedetector.find(world, blockposition);
 
                 if (shapedetector_shapedetectorcollection != null) {
+                    // CraftBukkit start - Use BlockStateListPopulator
+                    BlockStateListPopulator blockList = new BlockStateListPopulator(world);
                     for (int i = 0; i < shapedetector.getWidth(); ++i) {
                         for (int j = 0; j < shapedetector.getHeight(); ++j) {
                             ShapeDetectorBlock shapedetectorblock = shapedetector_shapedetectorcollection.getBlock(i, j, 0);
 
-                            world.setBlock(shapedetectorblock.getPos(), Blocks.AIR.defaultBlockState(), 2);
-                            world.levelEvent(2001, shapedetectorblock.getPos(), Block.getId(shapedetectorblock.getState()));
+                            blockList.setBlock(shapedetectorblock.getPos(), Blocks.AIR.defaultBlockState(), 2); // CraftBukkit
+                            // world.levelEvent(2001, shapedetectorblock.getPos(), Block.getId(shapedetectorblock.getState())); // CraftBukkit
                         }
                     }
 
@@ -73,6 +81,15 @@
                     entitywither.moveTo((double) blockposition1.getX() + 0.5D, (double) blockposition1.getY() + 0.55D, (double) blockposition1.getZ() + 0.5D, shapedetector_shapedetectorcollection.getForwards().getAxis() == EnumDirection.EnumAxis.X ? 0.0F : 90.0F, 0.0F);
                     entitywither.yBodyRot = shapedetector_shapedetectorcollection.getForwards().getAxis() == EnumDirection.EnumAxis.X ? 0.0F : 90.0F;
                     entitywither.makeInvulnerable();
+                    // CraftBukkit start
+                    if (!world.addFreshEntity(entitywither, SpawnReason.BUILD_WITHER)) {
+                        return;
+                    }
+                    for (BlockPosition pos : blockList.getBlocks()) {
+                        world.levelEvent(2001, pos, Block.getId(world.getBlockState(pos)));
+                    }
+                    blockList.updateList();
+                    // CraftBukkit end
                     Iterator iterator = world.getEntitiesOfClass(EntityPlayer.class, entitywither.getBoundingBox().inflate(50.0D)).iterator();
 
                     while (iterator.hasNext()) {
@@ -81,7 +98,7 @@
                         CriterionTriggers.SUMMONED_ENTITY.trigger(entityplayer, (Entity) entitywither);
                     }
 
-                    world.addFreshEntity(entitywither);
+                    // world.addFreshEntity(entitywither); // CraftBukkit - moved up
 
                     for (int k = 0; k < shapedetector.getWidth(); ++k) {
                         for (int l = 0; l < shapedetector.getHeight(); ++l) {