1. A programmatic method of caching message fragments during real-time messaging conversations, comprising: caching, in a message cache, an unsent outbound message fragment responding to a particular inbound message that arrives during a real-time messaging conversation, wherein: a representation of the cached message fragment is rendered in a sub-window of a graphical user interface (“GUI”) of the real-time messaging conversation; and the sub-window is generated responsive to the caching and occupies a portion of the GUI that is separate from an active window that renders a representation of each arriving inbound message and each response thereto; sending a response message, during the real-time messaging conversation after the caching, that responds to an inbound message that arrives after the particular inbound message; and subsequently recalling the cached message fragment from the message cache, responsive to a request, for responding to the particular inbound message during the real-time messaging conversation.