I am not exactly Web savvy, but I believe this is done by simply populating the window with text. If you inspect a single message in Chrome you can see that each message generates a list item looking something like this:
<li id="ember1551" class="ember-view chat-line">
<div class="indicator"></div>
<span class="timestamp float-left">6:03</span>
<span class="badges float-left"></span>
<span class="from" style="color:#1E90FF">Livewhiletrue</span>
<span class="colon">:</span>
<span class="message" style="undefined">my test message</span>
</li>
This is buried deep within a heriarchy, and I don’t know if this is generated in a consistent way. If it is you could traverse the DOM until you find the parent of these list items and generate a message like the above.