twitter/bootstrap — pull request #6169 by jlbruno (2012-12-09 02:23:20+00:00)
Title: Adding an 'expanded' class to click target

To help with styling elements that are targets for the collapse plugin, we required a class when the element was in the expanded state. Because the default state was collapsed, there was no difference in class on the target element from the default state through the first click to expand.

Resubmitting against master as suggested here:
https://github.com/twitter/bootstrap/pull/5938#issuecomment-11166599