Question: What computer accessory was the result of the work on human-computer interaction by the creator of hypertext?  

Context paragraphs:
Human-Computer Interaction Institute: The Human-Computer Interaction Institute (HCII) is a department within the School of Computer Science at Carnegie Mellon University (CMU) in Pittsburgh, Pennsylvania.  It is considered one of the leading centers of human-computer interaction research,

Douglas Engelbart: Douglas Carl Engelbart (January 30, 1925 – July 2, 2013) was an American engineer and inventor, and an early computer and Internet pioneer.  He is best known for his work on founding the field of human–computer interaction, particularly while at his Augmentation Research Center Lab in SRI International, which resulted in creation of the computer mouse, and the development of hypertext, networked computers, and precursors to graphical user interfaces.  These were demonstrated at The Mother of All Demos in 1968.  Engelbart's Law, the observation that the intrinsic rate of human performance is exponential, is named after him.

Human-centered computing: Human-centered computing (HCC) studies the design, development, and deployment of mixed-initiative human-computer systems.  It is emerged from the convergence of multiple disciplines that are concerned both with understanding human beings and with the design of computational artifacts.  Human-centered computing is closely related to human-computer interaction and information science.  Human-centered computing is usually concerned with systems and practices of technology use while human-computer interaction is more focused on ergonomics and the usability of computing artifacts and information science is focused on practices surrounding the collection, manipulation, and use of information.

Robert E. Kraut: Robert E. Kraut (born August 30, 1946) is an American social psychologist who studies human-computer interaction, online communities, internet use, group coordination, computers in organizations, and the role of visual elements in interpersonal communication.  He is a Herbert Simon Professor of Human-computer Interaction at the Human-Computer Interaction Institute at Carnegie Mellon University.

Stephen Brewster: Stephen Brewster FRSE is a Professor of Human-Computer Interaction in the Department of Computing Science at the University of Glasgow, UK.  His main research interest is Multimodal Human-Computer Interaction, sound and haptics and gestures.  Brewster received a PhD in the Human-Computer Interaction Group at the University of York.  He organized the Conference on Human-Computer Interaction with Mobile Devices and Services (MobileHCI) several times and contributed to several scientific books.

Chris Harrison (computer scientist): Chris Harrison is an English-born, American computer scientist and entrepreneur, working in the fields of human-computer interaction, machine learning and sensor-driven interactive systems.  He is an assistant professor at Carnegie Mellon University and director of the Future Interfaces Group within the Human-Computer Interaction Institute.  He has previously conducted research at AT&T Labs, Microsoft Research, IBM Research and Disney Research.  He is also the CTO and co-founder of Qeexo, a machine learning and interaction technology startup.

Keystroke-level model: In human–computer interaction, the keystroke-level model (KLM) predicts how long it will take an expert user to accomplish a routine task without errors using an interactive computer system.  It was proposed by Stuart K. Card, Thomas P. Moran and Allen Newell in 1980 in the "Communications of the ACM" and published in their book "The Psychology of Human-Computer Interaction" in 1983, which is considered as a classic in the HCI field.  The foundations were laid in 1974, when Card and Moran joined the Palo Alto Research Center (PARC) and created a group named Applied Information-Processing Psychology Project (AIP) with Newell as a consultant aiming to create an applied psychology of human-computer interaction.  The keystroke-level model is still relevant today, which is shown by the recent research about mobile phones and touchscreens (see Adaptions).

Augmented cognition: Augmented cognition is an interdisciplinary area of psychology and engineering, attracting researchers from the more traditional fields of human-computer interaction, psychology, ergonomics and neuroscience.  Augmented cognition research generally focuses on tasks and environments where human-computer interaction and interfaces already exist.  Developers, leveraging the tools and findings of neuroscience, aim to develop applications which capture the human user's cognitive state in order to drive real-time computer systems.  In doing so, these systems are able to provide operational data specifically targeted for the user in a given context.  Three major areas of research in the field are: Cognitive State Assessment (CSA), Mitigation Strategies (MS), and Robust Controllers (RC).  A subfield of the science, Augmented Social Cognition, endeavours to enhance the "ability of a group of people to remember, think, and reason."

Henry Lieberman: Henry Lieberman is an American computer scientist at the MIT Media Lab in the fields of programming languages, artificial intelligence and human-computer interaction.  He serves as a principal research scientist at the Media Lab and is the Director of the Software Agents Research group, which explores the intersection between artificial intelligence and human-computer interaction.

Purple Numbers: Purple Numbers is a suite of tools that allows one to address paragraphs of HTML content.  It was originally conceived by Douglas Engelbart for his creation of hypertext.  The name comes from the faint, purple numbers at the end of each paragraph, which represent the link to that paragraph.  The name was coined by Christina Engelbart, Doug's daughter.

Supporting paragraphs: Purple Numbers Douglas Engelbart
Answer: mouse