Autocomplete position incorrect when in a modal dialog

When using an autocomplete form field in a CTools modal dialog (for example a Fieldable Panel Pane with an Entity Reference field), the position of the autocomplete result is incorrect if the user has to scroll down in the modal dialog to see the field. The results appear near the top of the modal content dialog, rather than directly below the field.

Steps to reproduce

1. Create /modify a Fieldable Panel Pane bundle to contain an Entity Reference field.
2. On a panel page, add a new instance of the FPP.
3. Scroll the modal dialog down to the Entity Reference and start typing to activate the autocomplete.

Expected behavior

The autocomplete results should appear directly below the Entity Reference field.

Actual behavior

The autocomplete results appear part-way up the modal dialog content (see attached screenshot).

I've tested this on Drupal 7.x-dev in Firefox and Chrome on Ubuntu (DrupalPro).