Some browsers automagically provide the event in an event handler in a variable called `event` if it isn't already defined. When this part of bootstrap was originally written, this "feature" was used. When it later was tested in different browsers, it became apparent that the event wasn't always reliably available in an automagic `event` variable. Because a deadline was near, this was fixed by adding `var event = e;` instead of changing to `e.pageY ...` etc because it was quicker to write.