You're running into a sort of "safeguard" setting on our end you have no control over. In order to safeguard sessions from problematic scenarios (those which could cause a session to have pieces of components or other UI "stuck" on the page) we sometimes need to force behaviors. If you don't want the modal behavior, you can either forcibly limit the size of the modal (only available in the Nightly builds at this time), change the position of the Dropdown so that the options may display fully above or below it, or limit the number of options in the Dropdown.