Fixed dropdown enter key when searching

Closes #19741

CHANGELOG

@@ -37,6 +37,7 @@ v 8.10.0 (unreleased)
   - Make images fit to the size of the viewport !4810
   - Fix check for New Branch button on Issue page !4630 (winniehell)
   - Fix GFM autocomplete not working on wiki pages
+  - Fixed enter key not triggering click on first row when searching in a dropdown
   - Fix MR-auto-close text added to description. !4836
   - Support U2F devices in Firefox. !5177
   - Fix issue, preventing users w/o push access to sort tags !5105 (redetection)

app/assets/javascripts/gl_dropdown.js.coffee

@@ -35,6 +35,8 @@ class GitLabDropdownFilter
         $inputContainer.removeClass HAS_VALUE_CLASS
 
       if keyCode is 13
+        if @input.val() isnt "" and @options.enterCallback
+          @options.enterCallback(e)
         return false
 
       # Only filter asynchronously only if option remote is set
@@ -212,6 +214,9 @@ class GitLabDropdown
         callback: (data) =>
           currentIndex = -1
           @parseData data
+        enterCallback: (e) =>
+          @selectRowAtIndex(e, 0)
+
 
     # Event listeners