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

--- a/net/minecraft/world/level/block/BlockObserver.java
+++ b/net/minecraft/world/level/block/BlockObserver.java
@@ -14,6 +14,8 @@
 import net.minecraft.world.level.block.state.properties.BlockProperties;
 import net.minecraft.world.level.block.state.properties.BlockStateBoolean;
 
+import org.bukkit.craftbukkit.event.CraftEventFactory; // CraftBukkit
+
 public class BlockObserver extends BlockDirectional {
 
     public static final BlockStateBoolean POWERED = BlockProperties.POWERED;
@@ -41,8 +43,18 @@
     @Override
     public void tick(IBlockData iblockdata, WorldServer worldserver, BlockPosition blockposition, Random random) {
         if ((Boolean) iblockdata.getValue(BlockObserver.POWERED)) {
+            // CraftBukkit start
+            if (CraftEventFactory.callRedstoneChange(worldserver, blockposition, 15, 0).getNewCurrent() != 0) {
+                return;
+            }
+            // CraftBukkit end
             worldserver.setBlock(blockposition, (IBlockData) iblockdata.setValue(BlockObserver.POWERED, false), 2);
         } else {
+            // CraftBukkit start
+            if (CraftEventFactory.callRedstoneChange(worldserver, blockposition, 0, 15).getNewCurrent() != 15) {
+                return;
+            }
+            // CraftBukkit end
             worldserver.setBlock(blockposition, (IBlockData) iblockdata.setValue(BlockObserver.POWERED, true), 2);
             worldserver.scheduleTick(blockposition, (Block) this, 2);
         }