Steps to reproduce the issue

There are two ways to reproduce this:

• Switch to the Hathor template. I know it's out of date and never updated, but it serves this point for now. Go to Content > Articles > Add New Article. Click on the User icon next to the Created By field. The Modal window shows up (it should be based on MooTools). Click on any user in there and nothing happens.
• I've also noticed that both Protostar and Isis have layout overrides in /html/layouts/joomla/form/field/user.php. I do understand that they need overrides to use Bootstrap, but if I remove them I would expect them unstyled but functional. Removing overrides and re-doing the steps above will exhibit the same error.

Expected result

Clicking on a user should close the modal window and assign the user to the hidden field.

Actual result

Clicking on a user does nothing.

System information (as much as possible)

Joomla! 3.5.0 Stable [ Unicorn ] 21-March-2016 22:00 GMT

Additional comments

Seems to be caused by #5655
I've noticed this part https://github.com/joomla/joomla-cms/blob/staging/layouts/joomla/form/field/user.php#L50 that seems strange to me:

. ($required ? 1 : 0) . '&field={field-user-id}'

I'm guessing replacing it with:

. ($required ? 1 : 0) . '&field=' . htmlspecialchars($id, ENT_COMPAT, 'UTF-8')

Would do the trick. But I'm not sure if I'm missing something.