Safari Version 2.0.4 (419.3) and Opera Version 9.21 wont expand the Y scroll bar if an autocomplete (such as for the new Ajax User Picker dropdown) drops past the bottom of the screen. Essentially this means that entries beyond the window border cant be selected/seen by the user. The workaround is to use the 'old' user picker by clicking on the user icon next to the text field or upgrading to a real browser such as Firefox.

Note that this seems to have been fixed in Safari Version 3.0 (522.11.3).