We are using AutoCompleteExtender in our project and when checking in IE we don't see the full list but just first element. However, same site works just fine in Edge/Chrome. I did poking around and found that the list/dropdown is there but its somehow hiding behind and just one item hight of data is visible. here is how it looks.

IE:

Edge/Chrome:

IE when using element highligher :

Server side code

<ajaxTK:AutoCompleteExtender    ID="AutoCompleteExtenderGoToOrder" 
                            TargetControlID="txtGlobalOrderKeywords" 
                            runat="server" 
                            MinimumPrefixLength="3"
                            CompletionInterval="500" 
                            CompletionSetCount="20"
                            ServicePath="~/services/dataAjax.asmx"
                            ServiceMethod="GetGoToOrderList"  
                            UseContextKey="true"  
                            OnClientItemSelected="extenderOrderSelected"
                            CompletionListCssClass="autocompletegotoorder_completionListElement"
                            CompletionListItemCssClass="autocompletegotoorder_listItem" 
                            CompletionListHighlightedItemCssClass="autocompletegotoorder_highlightedListItem"
                            /> 

HTML

<DIV id=Header1_divGotoOrder class=gotoOrder>
     <SPAN id=Header1_lblGlobalGoToOrder class=topicHeader style="COLOR: white">Order:</SPAN> 
     <INPUT id=Header1_hdnGlobalOrderID type=hidden name=Header1$hdnGlobalOrderID> 
     <INPUT id=Header1_txtGlobalOrderKeywords style="WIDTH: 50%" value=metr name=Header1$txtGlobalOrderKeywords autocomplete="off">
     <UL id=Header1_AutoCompleteExtenderGoToOrder_completionListElem class=autocompletegotoorder_completionListElement style="WIDTH: 463px; POSITION: absolute; LEFT: 1405px; Z-INDEX: 1000; DISPLAY: none; TOP: 30px; VISIBILITY: hidden"></UL> 
</DIV>

CSS

.autocompletegotoorder_completionListElement
{
  padding-bottom: 0px;
  padding-top: 0px;
  padding-right: 0px;
  padding-left: 0px;
  position: absolute;
  z-index: 9999 !important;
 
  visibility: hidden;
  margin-left: -1px !important;
  margin-top: 0px !important;
  background-color: #F1F0F0;
  color: windowtext;
  border: buttonshadow;
  border-color: Black;
  border-width: 1px;
  border-style: solid;
  cursor: 'default';
  overflow: auto;
  height: 200px;
  text-align: left;
  list-style-type: none;
}
/* AutoComplete highlighted item */
.autocompletegotoorder_highlightedListItem
{
  list-style-type: none;
  font-family: Tahoma;
  font-size: 11px;
  color: White;
  text-decoration: none;
  background-color: Navy;
  cursor: pointer;
  cursor: hand;
  padding: 0px 0px 0px 3px;
  margin: 0px;
  text-align: left;
  line-height: 20px !important;
}
/* AutoComplete item */
.autocompletegotoorder_listItem
{
  line-height: 20px !important;
  background-color: #F1F0F0;
  padding: 0px 0px 0px 3px;
  margin: 0px;
  font-family: Tahoma;
  font-size: 11px;
  color: black;
  cursor: default;
  text-decoration: none;
  text-align: left;
}