But it doesn't always work this way, so you never know what to expect. But it can't always do that, since sometimes it is behaving like an edit text. In those cases, when you type characters, the Combo-Box instead just places your new text over the selection. But there seems to be a separate flag on whether you can add to the list or not. And always remember to NOT hit return, because that may just dismiss the dialog.
Some people think that because the control does some complex behaviors, that it is good.
After all, it solves a problem. Well solving a problem, and solving it well, are not always the same thing. Lets solve these problems using discrete controls and see if it is more clear, or if it is less clear. Remember, we already have a Menu, and Edit Text, and a List Box controls. They all do their respective functions clearer and better than a Combo-Box.