search: Fix display of suggestion box when search bar empty.

When search bar is empty and we've reached that state
by using the `backspace` key. There are no suggestions
as there are when you select an empty search bar.

The cause of this was an explicit prevention of this
suggestion box in `typeahead.js` so that the
`backspace` key is free to interact with the other
elements.

The fix here is to add an optional `hideOnEmpty` option
so that if we want this suggestion box to appear we can
set this option to `false` and this behavior will be
prevented.

This option is enabled for the search input when pills are not
enabled.

Fixes: #25062.

web/src/search.js

@@ -93,6 +93,9 @@ export function initialize() {
         parentElement: "#searchbox_legacy",
         items: search_suggestion.max_num_of_search_results,
         helpOnEmptyStrings: true,
+        // With search pills, the contenteditable input will be empty
+        // even if some pills are present.
+        hideOnEmpty: page_params.search_pills_enabled,
         naturalSearch: true,
         highlighter(item) {
             const obj = search_map.get(item);

web/third/bootstrap-typeahead/typeahead.js

@@ -487,8 +487,13 @@ import {get_string_diff} from "../../src/util";
 
         default:
           var hideOnEmpty = false
-          if (e.keyCode === 8 && this.options.helpOnEmptyStrings) { // backspace
-            hideOnEmpty = true
+          // backspace
+          if (e.keyCode === 8 && this.options.helpOnEmptyStrings) {
+            // Support for inputs to set the hideOnEmpty option explicitly to false
+            // to display typeahead after hitting backspace to clear the input.
+            if (typeof(this.options.hideOnEmpty) === undefined || this.options.hideOnEmpty) {
+              hideOnEmpty = true;
+            }
           }
           this.lookup(hideOnEmpty)
       }