Thank you. Your hints correct me. I found that the extra width which cause the display of horizontal bar is vertical scrollbar.
However, no matter what width value I set, the additional vertical bar will cause additional horizontal bar. For example, I see application width is 1000. All child is less than 800. It still has additional horizontal bar. One solution is to set vertical scrollbar to "on". Then, everything ok. However, I want the bar displays when it needs.
