Thanks @jollyolbrits for debugging, fix the issue with ground not being sent properly for movement.

Fixes anti-cheat issues.
Placement issues.
Probably 100 billion other issues.
Fix me being an idiot and changing numbers.

src/main/java/us/myles/ViaVersion/protocols/protocol1_9to1_8/PlayerMovementMapper.java

@@ -3,6 +3,7 @@ package us.myles.ViaVersion.protocols.protocol1_9to1_8;
 import us.myles.ViaVersion.api.PacketWrapper;
 import us.myles.ViaVersion.api.remapper.PacketHandler;
 import us.myles.ViaVersion.api.remapper.PacketRemapper;
+import us.myles.ViaVersion.api.type.Type;
 import us.myles.ViaVersion.protocols.protocol1_9to1_8.storage.MovementTracker;
 
 public class PlayerMovementMapper extends PacketRemapper {
@@ -11,7 +12,12 @@ public class PlayerMovementMapper extends PacketRemapper {
         handler(new PacketHandler() {
             @Override
             public void handle(PacketWrapper wrapper) throws Exception {
-                wrapper.user().get(MovementTracker.class).incrementIdlePacket();
+                MovementTracker tracker = wrapper.user().get(MovementTracker.class);
+                tracker.incrementIdlePacket();
+                // If packet has the ground data
+                if (wrapper.is(Type.BOOLEAN, 0)) {
+                    tracker.setGround(wrapper.get(Type.BOOLEAN, 0));
+                }
             }
         });
     }

src/main/java/us/myles/ViaVersion/protocols/protocol1_9to1_8/ViaIdleThread.java

@@ -4,24 +4,31 @@ import io.netty.channel.ChannelHandlerContext;
 import org.bukkit.scheduler.BukkitRunnable;
 import us.myles.ViaVersion.api.data.UserConnection;
 import us.myles.ViaVersion.protocols.base.ProtocolInfo;
-import us.myles.ViaVersion.protocols.protocol1_9to1_8.Protocol1_9TO1_8;
 import us.myles.ViaVersion.protocols.protocol1_9to1_8.storage.MovementTracker;
 import us.myles.ViaVersion.util.PipelineUtil;
 import us.myles.ViaVersion.util.ReflectionUtil;
 
+import java.lang.reflect.Field;
 import java.util.Map;
 import java.util.UUID;
 
 public class ViaIdleThread extends BukkitRunnable {
     private final Map<UUID, UserConnection> portedPlayers;
     private final Object idlePacket;
+    private final Object idlePacket2;
 
     public ViaIdleThread(Map<UUID, UserConnection> portedPlayers) {
         this.portedPlayers = portedPlayers;
         try {
             Class<?> idlePacketClass = ReflectionUtil.nms("PacketPlayInFlying");
             idlePacket = idlePacketClass.newInstance();
-        } catch (InstantiationException | IllegalArgumentException | IllegalAccessException | ClassNotFoundException e) {
+            idlePacket2 = idlePacketClass.newInstance();
+
+            Field flying = idlePacketClass.getDeclaredField("f");
+            flying.setAccessible(true);
+
+            flying.set(idlePacket2, true);
+        } catch (NoSuchFieldException | InstantiationException | IllegalArgumentException | IllegalAccessException | ClassNotFoundException e) {
             throw new RuntimeException("Couldn't find/make player idle packet, help!", e);
         }
     }
@@ -35,8 +42,11 @@ public class ViaIdleThread extends BukkitRunnable {
                     ChannelHandlerContext context = PipelineUtil.getContextBefore("decoder", info.getChannel().pipeline());
                     if (info.getChannel().isOpen()) {
                         if (context != null) {
-                            context.fireChannelRead(idlePacket);
-
+                            if (info.get(MovementTracker.class).isGround()) {
+                                context.fireChannelRead(idlePacket2);
+                            } else {
+                                context.fireChannelRead(idlePacket);
+                            }
                             info.get(MovementTracker.class).incrementIdlePacket();
                         }
                     }

src/main/java/us/myles/ViaVersion/protocols/protocol1_9to1_8/packets/WorldPackets.java

@@ -232,12 +232,8 @@ public class WorldPackets {
                     @Override
                     public void handle(PacketWrapper wrapper) throws Exception {
                         int status = wrapper.get(Type.UNSIGNED_BYTE, 0);
-                        if (status == 5)
+                        if (status == 4)
                             wrapper.cancel();
-                        if(status > 5){
-                            wrapper.set(Type.UNSIGNED_BYTE, 0, (short) (status - 1));
-                        }
-                        System.out.println("SENDING ID: " + wrapper.get(Type.UNSIGNED_BYTE, 0));
                     }
                 });
                 // Blocking
@@ -245,7 +241,7 @@ public class WorldPackets {
                     @Override
                     public void handle(PacketWrapper wrapper) throws Exception {
                         int status = wrapper.get(Type.UNSIGNED_BYTE, 0);
-                        if (status == 5) {
+                        if (status == 4) {
                             EntityTracker entityTracker = wrapper.user().get(EntityTracker.class);
                             if (entityTracker.isBlocking()) {
                                 entityTracker.setBlocking(false);

src/main/java/us/myles/ViaVersion/protocols/protocol1_9to1_8/storage/MovementTracker.java

@@ -1,6 +1,7 @@
 package us.myles.ViaVersion.protocols.protocol1_9to1_8.storage;
 
 import lombok.Getter;
+import lombok.Setter;
 import us.myles.ViaVersion.api.data.StoredObject;
 import us.myles.ViaVersion.api.data.UserConnection;
 
@@ -9,6 +10,9 @@ public class MovementTracker extends StoredObject {
     private static final long IDLE_PACKET_LIMIT = 20; // Max 20 ticks behind
     @Getter
     private long nextIdlePacket = 0L;
+    @Getter
+    @Setter
+    private boolean ground = true;
 
     public MovementTracker(UserConnection user) {
         super(user);