I have a page that contains a ModalWindow. That ModalWindow contains an AutoCompleteTextField.
If the main page has a lot of content, the AutoCompleteTextField only works correctly if the page is scrolled to the top.
You can see this easily by scrolling the page up and down while the ModalWindow is open and the AutoCompleteTextField is showing its dropdown.
I believe what is happening is that the dropdown has an absolute position relative to the Page, rather than relative to the ModalWindow. When you scroll the page down, the dropdown remains in the same position on the page (and hence scrolls off the top), while the ModalWindow stays in the same position on the screen.