When the content of the body is not much the "Overlay" does not fill IE's window

it only happens in IE

a solution would be to include

body{
	height:100%;
}

either in thickbox.css
or the main drupal css file

so that body occupies the whole height of the browser window

its probably an issue that thickbox has itself and not the drupal module but documenting it just in case anyone is interested