[frik]

Don't down vote him. More often than not those are just normal refactoring with PR attempts to get rid of old license info. See IE7, it got rid of Mosaic license info yet even IE11 and early Edge contains web render code and mouse behaviors that can be traced back to Mosaic web browser, where it forked off. The same is with game engines, Valve's Source engine 2 started as a fork of ID's Quake 1 engine and it's still evident even in 2017. Or Dunia engine from Ubisoft traces back to FarCry 1 engine from Crytek. Or Call of Duty engine originates in Quake 3 engine from ID. Or in WinWord 2010 originates in WinWord2 code from 1990, with code that predates threads and uses fibers (known as coroutines nowadays in other languages).

[NotSammyHagar]

it was far from a simple refactoring. we wrote almost every piece of it new and from scratch.

[justin66]

I'm pretty skeptical that Edge ever contained any old Spyglass code. If they kept the licensing disclaimer in those early versions, I suspect it was because proving that none of the old code was there wasn't a priority for anyone.

[pjmlp]

Unless I stop understanding English, all those examples are still a rewrite, writing something anew, doesn't mean throwing 100% of the old stuff away.

[djsumdog]

And these can also be explained by unit tests .. you keep the old unit tests and rewrite everything so those tests pass. Sometimes that means getting some of the same weird quirks or gotchas, but hopefully refactored so they are all in one place and clearly documented.

[BarkMore]

IE4 was a rewrite, not a refactor job.