Hopefully a fix for #12364

* register the callback before invoking the toggling
* listen for `hidden` instead of `hide`
* account for the possibility of the associated `show` event being preventDefault-ed

Adds relevant unit tests.

js/modal.js

@@ -268,10 +268,13 @@
 
     if ($this.is('a')) e.preventDefault()
 
-    Plugin.call($target, option, this)
-    $target.one('hide.bs.modal', function () {
-      $this.is(':visible') && $this.trigger('focus')
+    $target.one('show.bs.modal', function (showEvent) {
+      if (showEvent.isDefaultPrevented()) return  // only register focus restorer if modal will actually get shown
+      $target.one('hidden.bs.modal', function () {
+        $this.is(':visible') && $this.trigger('focus')
+      })
     })
+    Plugin.call($target, option, this)
   })
 
 }(jQuery);