<p><strong>I think this is a great idea.</strong></p>

<p>That feature is already used in the "click to expand/collapse this section" organization of all the FAQs.  Being able to mark up a reply to do the same thing could really streamline all the SE sites.  (I would love to collapse large blocks of code when reading through some of the lengthier replies, for instance.)</p>

<p>In fact, one approach could be to make some existing forms of marked up text automatically (and retroactively) collapsible.  This could include code blocks, quotations, images, and (maybe) lines of TeX.  It might even extend to blocks that are delimited by heading markup (even though that has rarely been used so far).</p>
