Merge pull request #18393 from twbs/dropdown-bitwise-less-tricksy

dropdown.js: Use more straightforward phrasing for index bound check

js/src/dropdown.js

@@ -250,7 +250,7 @@ const Dropdown = (($) => {
         index++
       }
 
-      if (!~index) {
+      if (index < 0) {
         index = 0
       }