MDL-59365 autocomplete: Keep the suggestions open

If the list allows multiple selections, don't auto-close the suggestions list
every time something is chosen.

lib/amd/src/form-autocomplete.js

@@ -450,10 +450,18 @@ define(['jquery', 'core/log', 'core/str', 'core/templates', 'core/notification']
         updateSelectionList(options, state, originalSelect);
         // Notifiy that the selection changed.
         notifyChange(originalSelect);
-        // Clear the input element.
-        inputElement.val('');
-        // Close the list of suggestions.
-        closeSuggestions(state);
+
+        if (!options.multiple) {
+            // Clear the input element.
+            inputElement.val('');
+            // Close the list of suggestions.
+            closeSuggestions(state);
+        } else {
+            // Focus on the input element so the suggestions does not auto-close.
+            inputElement.focus();
+            // Remove the last selected item from the suggestions list.
+            updateSuggestions(options, state, inputElement.val(), originalSelect);
+        }
     };
 
     /**