Fix slime blocks in the upside down

src/main/java/com/minelittlepony/unicopia/mixin/gravity/MixinPistonBlockEntity.java

@@ -0,0 +1,46 @@
+package com.minelittlepony.unicopia.mixin.gravity;
+
+import org.spongepowered.asm.mixin.Mixin;
+import org.spongepowered.asm.mixin.Shadow;
+import org.spongepowered.asm.mixin.injection.At;
+import org.spongepowered.asm.mixin.injection.Inject;
+import org.spongepowered.asm.mixin.injection.callback.CallbackInfo;
+
+import net.minecraft.block.entity.PistonBlockEntity;
+import net.minecraft.entity.Entity;
+import net.minecraft.util.math.BlockPos;
+import net.minecraft.util.math.Box;
+import net.minecraft.util.math.Direction;
+import net.minecraft.world.World;
+
+@Mixin(PistonBlockEntity.class)
+abstract class MixinPistonBlockEntity {
+    @Shadow
+    private static Box offsetHeadBox(BlockPos pos, Box box, PistonBlockEntity blockEntity) {
+        return box;
+    }
+
+    @Shadow
+    private static boolean canMoveEntity(Box box, Entity entity, BlockPos pos) {
+        return false;
+    }
+
+    @Shadow
+    private static void moveEntity(Direction direction, Entity entity, double distance, Direction movementDirection) {
+
+    }
+
+    @Inject(method = "moveEntitiesInHoneyBlock", at = @At("TAIL"))
+    private static void moveEntitiesInHoneyBlock(World world, BlockPos pos, float ticks, PistonBlockEntity tile, CallbackInfo info) {
+        Direction direction = tile.getMovementDirection();
+        if (!direction.getAxis().isHorizontal()) {
+            return;
+        }
+        double blockY = tile.getPushedBlock().getCollisionShape(world, pos).getMin(Direction.Axis.Y);
+        Box box = offsetHeadBox(pos, new Box(0, blockY - 1.5000010000000001F, 0, 1, blockY, 1), tile);
+        double distance = ticks - tile.getProgress(1);
+        for (Entity entity2 : world.getOtherEntities(null, box, entity -> canMoveEntity(box, entity, pos))) {
+            moveEntity(direction, entity2, distance, direction);
+        }
+    }
+}

src/main/resources/unicopia.mixin.json

@@ -58,6 +58,7 @@
     "gravity.MixinLivingEntity",
     "gravity.MixinMobEntity",
     "gravity.MixinWorld",
+    "gravity.MixinPistonBlockEntity",
     "gravity.MixinServerWorld",
     "gravity.MixinServerPlayerEntity",
     "gravity.MixinServerPlayNetworkHandler",