Steps to reproduce the issue

• For the sake of using SqueezeBox (MooTools based) instead of the Bootstrap Modal, change the Administrator template to Hathor. It's damn ugly, I know :)
• Make sure you're using TinyMCE as your Editor.
• Go to Content > Articles > Add New Article.
• In the Publishing area, click on the "User" icon so you can select a user from the modal.
• The modal shows up. Click outside the modal or on the X button to close it.

Expected result

Modal is closed.

Actual result

Modal is not closed - Firebug shows:

too much recursion
SqueezeBox.close = function () {

System information (as much as possible)

Joomla! 3.6.0, Firefox 47.0.1

Additional comments

Seems to be caused by the code TinyMCE replaces (#11048) in the SqueezeBox.close() function. Will take a deeper look into the code, see if I can come up with a fix and PR.