Change to use jQuery's function for checking if a single element is visible.
Change to use jQuery's function for checking if a single element is visible.

To achieve the same comprehensive check of visibility as the original
$el.is(":visible") code, we use jQuery's function for checking if a
single element is visible.  Which is a different method to
$el.is(":visible") which looks for $el in the array of elements that
match :visible from the entire DOM

ichord/At.js,ichord/At.js