If I open the image uploader, the background goes darker.
Now that is by design, and if or click outside the upload box, the box goes, and the background disappears.
However, if you press Esc, only the box goes - the background stays the same. I think this is a recent bug - I always used to use Esc, and it worked perfectly.
Chrome 39.0.2171.95 on Ubuntu GNOME 14.04.1