The fact that Xerox has not tried to sue Apple is the clearest indication that the GUI concept wasn't technically stolen. But that doesn't change the fact that idea wasn't Apple's (could not be copyrighted by Apple) and therefore couldn't be stolen from Apple by Microsoft, as alleged in Apple's lawsuit.
There were non-disclosure agreements on part of the lawsuit, so details aren't fully known.
Xerox tried to sue Apple but it went nowhere. Apple did improve quite a bit on the basic Xerox concept, and managed to shoehorn it into a relatively inexpensive computer. Only large corporations could afford what Xerox eventually made from the technology.
OTOH, Microsoft tried to copy Apple and produced some quite inferior copies that had problems running well on current hardware. BTW, Microsoft did use Apple code in Windows without permission. Microsoft had to pay out a lot for that.
In actual fact, Vic, in 1990, after the Mac came out and Apple was suing Microsoft over the "look and feel" issues of Windows, a new CEO at Xerox initiated a similar lawsuit against Apple based on the myth that Apple had "stolen" the GUI from Xerox's PARC during those visits. The judge tossed Xerox's suit out for two reasons 1) Apple produced the signed documents outlining the Apple common stock in exchange for PARC visits and rights for ideas learned during said visits agreement; and, 2) the technicality that Xerox's suit was filed far too late and any statute of limitations for damages clock had started ticking in January 1983 with the release of the Apple Lisa. However, the judge ordered sanctions on Xerox's legal counsel for bringing the case in light of the signed agreements for not doing due diligence and wasting the court's time.
Attempts have been made to bury these facts. An article written by Yvonne Lee—quoted clumsily in the Wikipedia article on the Apple v Microsoft suit that initiated the Xerox suit—goes as far as manufacturing a statement from a "Xerox spokesman" saying that Xerox did indeed own some Apple stock they had purchased in August of 1979 "as an investment" which they later sold, to explain away the stock that Xerox got in the agreement. Lee's article also quotes others to denigrate the accounts of top Macintosh people such as Jef Raskin about the PARC visits. There are some RED FLAGS with the Lee article that reveal it to be fraudulent. Xerox could NOT have purchased common stock in Apple as an investment in August of 1979. Apple's IPO did not take place until December 12, 1980. The Lee article prompted two Mac developers to document the events, in detail, and they produced documents and photographic evidence proving her article to be FUD.
Ideas cannot be copyrighted. Specific design elements and implementations can be patented (though "look and feel" patents are controversial), and those elements were introduced on the Mac, duplicated in Windows, and absent on the Alto/Star.