Did the computer scientist at Xerox really develop the first graphical user interface, later showing this to both Steve Jobs and Bill Gates?
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I suspect this is going to degenerate into an argument about what words mean, but back in 1965 the IBM 2250 graphics terminal had the equivalent of a mouse (actually a light pen) and a keypad with 32 clickable buttons that could be programmed to do anything (and have their functionality redefined depending on the context of what was being displayed on the screen).
Of course people wrote software for it that had a "graphical user interface" - though Xerox didn't invented the term until 1973, which is often considered the beginning of the concept of GUI.
It doesn't meet the OP's criterion though, because it was a commercial product that was actually sold to customers.
I doubt they showed the pre-release version to Steve Jobs, since he wouldn't have reached his 10th birthday at the time.