Using the autocompleteextender over a popupextender or any other control with z-index property greater than zero.

This issue can be fixed easily using a little of CSS Code we need to define 3 CSS clases one for the CompletionListCssClass property of the extender, one more for the CompletionListItemCssClass and the last one for the CompletionListHighlightedCssClass attribute.

******************** Autocomplete extender control *******************************

CompletionListCssClass="autocomplete_completionListElement" CompletionListItemCssClass="autocomplete_listItem" CompletionListHighlightedItemCssClass="autocomplete_highlightedListItem"

*********************** CSS *********************************

    /* AutoComplete highlighted item */
    .autocomplete_completionListElement
    {
        margin: 0px !important;
        z-index:99999 !important;
        background-color: ivory;
        color: windowtext;
        border: buttonshadow;
        border-width: 1px;
        border-style: solid;
        cursor: 'default';
        overflow: auto;
        height: 200px;
        text-align: left;
        left: 0px;
        list-style-type: none;
    }
    /* AutoComplete highlighted item */
    .autocomplete_highlightedListItem
    {
        z-index:99999 !important;
        background-color: #ffff99;
        color: black;
        padding: 1px;
        cursor:hand;
    }
    /* AutoComplete item */
    .autocomplete_listItem
    {
        z-index:99999 !important;
        background-color: window;
        color: windowtext;
        padding: 1px;
        cursor:hand;
    }