fix: entity onGround applies correctly (#2059)

src/main/java/net/minestom/server/collision/PhysicsUtils.java

@@ -47,7 +47,7 @@ public final class PhysicsUtils {
     private static @NotNull Vec updateVelocity(@NotNull Pos entityPosition, @NotNull Vec currentVelocity, @NotNull Block.Getter blockGetter, @NotNull Aerodynamics aerodynamics,
                                                boolean positionChanged, boolean entityFlying, boolean entityOnGround, boolean entityNoGravity) {
         if (!positionChanged) {
-            if (entityOnGround || entityFlying) return Vec.ZERO;
+            if (entityFlying) return Vec.ZERO;
             return new Vec(0, entityNoGravity ? 0 : -aerodynamics.gravity() * aerodynamics.verticalAirResistance(), 0);
         }
 

src/test/java/net/minestom/server/entity/EntityPhysicsIntegrationTest.java

@@ -0,0 +1,32 @@
+package net.minestom.server.entity;
+
+import net.minestom.server.coordinate.Pos;
+import net.minestom.server.instance.block.Block;
+import net.minestom.testing.Env;
+import net.minestom.testing.EnvTest;
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.*;
+
+@EnvTest
+public class EntityPhysicsIntegrationTest
+{
+    @Test
+    public void onGround(Env env) {
+        var instance = env.createFlatInstance();
+        instance.setBlock(1, 40, 1, Block.STONE);
+
+        var entity = new Entity(EntityTypes.ZOMBIE);
+        entity.setInstance(instance, new Pos(1, 41, 1)).join();
+        env.tick();
+
+        // Entity shouldn't be on ground because it intitially spawns in with onGround = false
+        // and a velocity of 0, it'll take 1 entity tick for gravity to be applied to their velocity
+        // and a downward block collision to occur
+        assertFalse(entity.onGround);
+        for (int i = 0; i < 10; i++) {
+            env.tick();
+            assertTrue(entity.onGround, "entity needs to be grounded on tick: " + entity.getAliveTicks());
+        }
+    }
+}