Update select2.js

above function doesn't work great

select2.js

@@ -1218,8 +1218,8 @@ the specific language governing permissions and limitations under the Apache Lic
             };
 
             if (above) {
-                css.bottom = windowHeight - offset.top;
-                css.top = 'auto';
+                css.top = offset.top - dropHeight;
+                css.bottom = 'auto';
                 this.container.addClass("select2-drop-above");
                 $dropdown.addClass("select2-drop-above");
             }