You may have noticed while experimenting with popups that they contain a button in the upper right that can be used to toggle it between its default state as a callout window connected to the clicked feature and a docked state. Docking the popup can be especially useful when it contains a lot of information or if the app is used on mobile devices.
Customizing the docking behavior is done by working with the Popup object’s dockOptions property. This property can be set using an object with one or more of the following properties: buttonEnabled, position and breakpoint. The buttonEnabled property can be set to False if you’d like to remove the docking button from the popup interface. The position property can be set to one of six allowed values ('top-right', 'top-left', 'top-center', 'bottom-right', 'bottom-left', and 'bottom-center'). The breakpoint property can be set to an object that defines the dimensions at which the popup should be docked automatically. See the Esri's Popup Docking sample if you’re interested in learning more on this topic.