A control with an associated
which will appear above or below the control. See
PopupButton for a typical control that uses
a button to invoke the popup.
See also ComboBox for a control that
associates a popup with a text box.
The control attempts to position the popup such that it doesn't cover up the control.
E.g., if the control is near the bottom of the viewport, and there is room to show the
popup above the control, the popup will appear above the control.