Henry's got me on ignore for being an Apple Hater, but for the sake of correctness, I'm gonna go on record by stating he's not entirely right. Xerox itself didn't see much of a future in computers, let alone GUIs, but PARC did, and was able to convince them to release the Star, which sold for...well, not great, it started at $10,000 USD back in '81, but it was on the market.
Apple licensed the rights to the GUI from Xerox around that same time, and expanded upon it, eventually releasing two products, the Lisa, which was a flop for the very same reason the Star was, and the Macintosh, which sold fairly well it's first year, but tapered off after that. The first truly commercially successful GUI based system was actually the Amiga 500, which pretty much blew everyone away back when it came out, and sold a ton for its time.
And yeah, Apple did come up with some interesting GUI concepts that weren't covered by the Star, though not as much as some people tend to believe around here. Their biggest contribution to the GUI was drag 'n drop, the universal menu, and the trash can. Impressive stuff, but they hardly deserve credit for graphical user interfaces because of it.
And yeah, the Star did have overlapping windows before the Mac.
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I wanted to. Hell, I tried to. But I was so damn hungry, man. And they knew it!