Fix Fomantic UI dropdown icon bug when there is a search input in menu (#27225) (#27228)

Backport #27225 by @wxiaoguang

Fix #27224

And add the case to the devtest page.

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

templates/devtest/gitea-ui.tmpl

@@ -171,6 +171,7 @@
 				<span class="text">simple</span>
 				{{svg "octicon-triangle-down" 14 "dropdown icon"}}
 				<div class="menu">
+					<div class="ui icon search input"><i class="icon">{{svg "octicon-search"}}</i><input type="text" value="search input in menu"></div>
 					<div class="item">item</div>
 				</div>
 			</div>

web_src/js/modules/fomantic.js

@@ -16,6 +16,9 @@ export function initGiteaFomantic() {
   $.fn.dropdown.settings.fullTextSearch = 'exact';
   // Do not use "cursor: pointer" for dropdown labels
   $.fn.dropdown.settings.className.label += ' gt-cursor-default';
+  // The default selector has a bug: if there is a "search input" in the "menu", Fomantic will only "focus the input" but not "toggle the menu" when the "dropdown icon" is clicked.
+  // Actually, the "search input in menu" shouldn't be considered as the dropdown's input
+  $.fn.dropdown.settings.selector.search = '> input.search, :not(.menu) > .search > input, :not(.menu) input.search';
   // Always use Gitea's SVG icons
   $.fn.dropdown.settings.templates.label = function(_value, text, preserveHTML, className) {
     const escape = $.fn.dropdown.settings.templates.escape;