I find my Dropdownchoice missing while scrolling the vertical scroll bar. After I look into the codes and compare with other web pages, I find the dropdownchoice component only dispeared with Tinymce component in the same page.
Both Dropdownchoice and Timymce can work well without a small length web page. But when the length of web page large enough and the vertical scroll bar showup , the strange thing happend. When I move the vertical scroll bar, the Dropdownchoice gets dispeared. I have removed all the CSS used in my web page,so I'm so sure this problem is lead by Tinymce CSS. Does anyone find this strange condition happend?and how do you resolve this problem? thanks.