Popup: do not call blur if the target is the body

In IE 10 (and possibly older versions of IE) when there is a single tab open
and a popup is displayed and there is another window on the same monitor,
calling target.blur() can cause the browser to go to the background because the
body element is the targetElement.  Ensuring the element is not the body element
before calling blur() fixes this issue.

Fixes gh-7639
Closes gh-7298