Hey there,
Something is really strange.
Once disabled JCH plugin, now always first time not working but when you refresh it, it will work fine.
You can check my video link here :
http://screencast.com/t/68zSKl4o7q
By the way, after troubleshoot long hour ago, I still unable to find out which part conflict this then causing the scrollbar missing issue.
But I realised that your current template conflict with the Easydiscuss bbcode editor, this is what I test just now :
1. Switch to Joomla default template
"Protostar" or "JSN Pixel PRO", that scrollbar will always appeared.
2. Switch to other editor example : Tiny-MCE editor + your current template
(JSN Epic PRO), that scrollbar will always appeared.
May i know do you consider switch your editor to use WYSIWYG editor e.g. JCE, Tiny-MCE , or switch to another template e.g. JSN Pixel PRO?