Both Apple and Xerox were working on GUIs simultaneously. Apple had hired an ex-professor from California State University San Diego who had done pioneering work in windowing and mouse driven GUIs. He had given several lectures at PARC before going to work at Apple that had sparked their interest in GUIs and he was now the head of Apple's GUI and Mac project. Perhaps you've heard of him? His name is Jef Raskin. HE was later miffed that Jobs took over his Mac project... But later came to realize the genius that Jobs brought to the project through simplification. It was Jef Raskin who is the real father of the GUI at both Apple and PARC... and HE suggested that Jobs visit PARC to see what they were doing and put Jobs in contact with Xerox management to negotiate a deal to visit AND USE WHAT THEY SAW on the visits and it was Jobs and Raskin who went on the first eight hour visit. They lifted NOTHING! In fact, Raskin made suggestions to the PARC engineers on their visit for improvements on their work! In many ways it was a two way street.
On the other hand, Gates did sign an NDA, and violated it in many ways... But the Sugar-water salesman who was CEO of Apple had little tech understanding an wrote a very poor contract with Microsoft and the the judge in the case was a Luddite with even less understanding of software copyrights. . . who ruled that a limited license for Windows 1, was an unlimited license for all subsequent versions of Windows dispite explicit limiting language. He said "Windows is Windows!" ignoring the complete re-write and change that took place from Windows 1 to Windows95. He found only a few similar icons such as the Trashcan were protected. Idiot.
Incidentally, Microsoft did lose a MAJOR patent and software copyright infringement case to Apple in 1997... It cost the more than $150,000,000! It had the potential of costing them billions, but Steve Jobs found a way to save face for them so that both companies won.
No doubt yours is the definitive accounting...the short version at least. I like how Xerox got one million pre-IPO shares of Apple stock in payment. What great days those were for pioneers with hi-IQs and guts!