1. A method comprising: outputting, by one or more computing devices and for display, a first interface area in which an editable document is displayed and a second interface area outside of the first interface area; in response to a paste command, inserting copied content into the editable document; identifying a plurality of candidate sources for the copied content, each candidate source being a distinct source of the copied content, at least some of the candidate sources being identified based on documents concurrently open in a third interface area outside of the first interface area and the second interface area; outputting, in the second interface area, a plurality of interface elements, wherein each respective interface element corresponds to a respective candidate source for the copied content; receiving, from a user, a selection of one of the interface elements; and generating, by the one or more computing devices, citation information in the editable document based on the candidate source associated with the selected one of the interface elements.
4. The method of claim 1 , wherein the copied content is received from an inter-application clipboard.