Normally you use Sprite.buttonMode = true. Mousing over a TextFields brings up a potentially annoying caret. To get the buttonMode working on a TextField, place the TextField into a Sprite and:
Sprite.buttonMode = true;
Sprite.mouseChildren = false;
If you’re using a and Flash Actionscript 2.0 Loader to import another .swf file to the stage and your ComboBox/dropdown is no longer working – all you need to do is import the ComboBox component to the library of the .fla file that contains the loader i.e. drag it to the stage then delete it.
It’s basically a bug though we’ll be polite and call it a quirk =)