The default behavior of this component is to shows the dropdown right after it get focus(e.g, you clicked in the inputfield). It covered the (important) thing underneath. The problem is the dropdown cannot be dissmissed by any clicking other place or an ESC hit, it stay there until you deliberately input some wrong words or click an item which leads to unwanted result.

You can check the result by the demo on dartpad which is just a copy-paste from the offical document.

BTW, dartpad itself has problem to show the demo correctly. You have to click the run button at least twice to show all the stuff on screen. I don't know why.