With jQuery UI 1.8.4, when a dialog box is opened, there is some whitespace added at the end of the page (of the height of the dialog box). The whitespace is removed when the dialog box is closed.
If there is no scrollbar visible, reduce the size of the browser window until the vertical scrollbar appears.
Scroll down, and notice there is whitespace at the end of the page. Close the dialog box and notice the whitespace is now gone.
Using: jQuery 1.4.2 and custom jQuery UI 1.8.4 with only Draggable and Dialog components.
Happens with: Firefox 3.6, Opera 10.6, Chromium 5.0. No idea if Internet Explorer is affected.