Collapse: count `.collapsing` elements as actives; closes #13447

js/collapse.js

@@ -42,7 +42,7 @@
     this.$element.trigger(startEvent)
     if (startEvent.isDefaultPrevented()) return
 
-    var actives = this.$parent && this.$parent.find('> .panel > .in')
+    var actives = this.$parent && this.$parent.find('> .panel').children('.in, .collapsing')
 
     if (actives && actives.length) {
       var hasData = actives.data('bs.collapse')