I hate licensing issues :(
On happier news, XenForo has added something similar to their core in 1.2. There are two key differences that I see... firstly they handle spaces (can't remember if we do or not, though I seem to remember talking about it)
Secondly, and this is more of a matter for @Nao
: the auto suggester in XF also includes the user profile alongside the user name. Is this something we want to consider?
Thirdly, they don't worry quite so much about positioning for the suggester popup for mention purposes. It seems to align to the left of the window on the row directly below the cursor's current position. But they're now using Redactor rather than TinyMCE. It's much lighter than TMCE but I think it's still bigger than what we have - but it's probably a bit more reliable.
* Arantor thinks we really need to spend time with the WYSIWYG editor to make it a viable competitor... there is a reason everyone else uses off the shelf components (XF = Redactor, IPB = CKEditor, WordPress = TinyMCE, vB = CKEditor)