FIX: Autocomplete fadeout arrow key chat fix (#19600)

Autocomplete with fadeout was not scrolling on arrow
key press in chat, since the input is treated slightly
differently. We just need to find the fadeout div sooner.

Follow up to 64a7a2aac2

app/assets/javascripts/discourse/app/lib/autocomplete.js

@@ -377,6 +377,8 @@ export default function (options) {
       me.parent().append(div);
     }
 
+    fadeoutDiv = div.find(".hashtag-autocomplete__fadeout");
+
     if (isInput || options.treatAsTextarea) {
       _autoCompletePopper && _autoCompletePopper.destroy();
       _autoCompletePopper = createPopper(me[0], div[0], {
@@ -450,8 +452,6 @@ export default function (options) {
       top: mePos.top + pos.top - vOffset + borderTop + "px",
       left: left + "px",
     });
-
-    fadeoutDiv = div.find(".hashtag-autocomplete__fadeout");
   }
 
   function dataSource(term, opts) {