Jquery UI dialog background not growing when form loaded with ajax

When I load a form into a Jquery UI dialog, if the form is longer than the existing page, the background does not continue down the full length of the page.

If I use the resizer to shrink the dialog and then stretch it back again then the background stretches with it.

Just wondering if there is some kind of function I can call to refresh the background size after I've done the Ajax load?

UPDATE: I think开发者_StackOverflow I've found the answer, by changing .ui-widget-overlay to position:fixed it seems to solve the problem, just wondering if it will have any dodgy side-effects?