Combobox Examples: dropdown can be outside the viewport

[zcorpan]

Related to #1255

In the combobox examples, if the combobox is near the bottom of the viewport when the user focuses it and the dropdown is shown, the dropdown will be partially outside the viewport.

This can also happen if there is no content below the combobox, so the user can't scroll further until the dropdown is open.

I think this is typically dealt with by opening the dropdown upwards instead, or by scrolling the document so the dropdown is in view.