In fact Smalltalk-76 didn't have overlapping Windows, and the people at Apple had already come up with that before seeing the Smalltalk demo. Bill Atkinson describes that here. https://www.cnet.com/news/tracing-the-origins-of-the-macinto...

The screenshot in the article (which _does_ have overlapping windows) is evidently Smalltalk-80 because Smalltalk-76 used the "hollow colon" character in its message syntax where Smalltalk-80 uses the typographic colon.