MDL-78885 comboboxsearch: Improve user search and keyboard handling

- Prevent rerendering of the menu when it's already open to preserve the
  selected/highlighted option.
- Allow ArrowUp and ArrowDown to trigger the user search dropdown for
  improved keyboard navigation.

This commit ensures that the menu is not re-rendered when it's already
open, preserving the selected option to enhance the user experience.
Additionally, ArrowUp and ArrowDown keys now trigger the dropdown,
improving keyboard navigation and user interactions.

lib/amd/src/comboboxsearch/search_combobox.js

@@ -352,7 +352,11 @@ export default class {
             this.searchInput.removeAttribute('aria-activedescendant');
         }
         // User may have accidentally clicked off the dropdown and wants to reopen it.
-        if (e.target.closest(this.selectors.input) && this.getSearchTerm() !== '') {
+        if (
+            this.getSearchTerm() !== ''
+            && !this.getHTMLElements().searchDropdown.classList.contains('show')
+            && e.target.closest(this.selectors.input)
+        ) {
             await this.renderAndShow();
         }
     }

user/amd/src/comboboxsearch/user.js

@@ -212,6 +212,16 @@ export default class UserSearch extends search_combobox {
     keyHandler(e) {
         // Switch the key presses to handle keyboard nav.
         switch (e.key) {
+            case 'ArrowUp':
+            case 'ArrowDown':
+                if (
+                    this.getSearchTerm() !== ''
+                    && !this.searchDropdown.classList.contains('show')
+                    && e.target.contains(this.combobox)
+                ) {
+                    this.renderAndShow();
+                }
+                break;
             case 'Enter':
             case ' ':
                 if (e.target.closest(this.selectors.resetPageButton)) {