Normal Flex List control doesn't behave as expected when horizontalScrollPolicy is set to auto. The users are expected to maintain the maxHorizontalScrollPosition themselves. The given reason behind this behavior is 'performance'. If you are one of those users who don't care a little performance degradation please give the following custom component a try. ListEx derives from List and has code to handle to horizontal scroll bar when policy is set to auto.
The horizontalScrollForVisibleItems can be used to tweak performance for large data sets or data coming from a server.
If you find any bugs please let me know. I would be glad to fix it.