Fix MinestomThread local cache

src/main/java/net/minestom/server/thread/MinestomThread.java

@@ -33,7 +33,7 @@ public class MinestomThread extends Thread {
         final int requiredLength = index + 1;
         if (array.length < requiredLength) {
             Object[] temp = new Object[requiredLength];
-            System.arraycopy(temp, 0, temp, 0, array.length);
+            System.arraycopy(array, 0, temp, 0, index);
             array = temp;
             this.locals = array;
         }

src/test/java/thread/LocalThreadCache.java

@@ -0,0 +1,43 @@
+package thread;
+
+import net.minestom.server.thread.MinestomThread;
+import org.junit.jupiter.api.Test;
+
+import java.util.concurrent.atomic.AtomicBoolean;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+public class LocalThreadCache {
+
+    @Test
+    public void testLocalThreadCache() throws InterruptedException {
+        AtomicBoolean result = new AtomicBoolean(false);
+        var thread = new MinestomThread("name") {
+            @Override
+            public void run() {
+                final int dummy = -1;
+
+                var value = localCache(0, () -> 5);
+                assertEquals(5, value);
+
+                value = localCache(0, () -> dummy);
+                assertEquals(5, value);
+
+                value = localCache(1, () -> 7);
+                assertEquals(7, value);
+                assertEquals(5, localCache(0, () -> dummy));
+
+                value = localCache(2, () -> 5);
+                assertEquals(5, value);
+                assertEquals(7, localCache(1, () -> dummy));
+                assertEquals(5, localCache(0, () -> dummy));
+
+                result.set(true);
+            }
+        };
+        thread.start();
+        thread.join(1500);
+        assertTrue(result.get(), "Thread didn't start");
+    }
+}