There are gray areas but I do not think you are in one.
> and then later adapted and expanded upon that code (or even started over, with the knowledge of what you learned from others' work)
These are extremely different scenarios. Starting with a copyrighted material and modifying it is not at all the same as reading material and starting over. The first is violating copyright, the second is a derivative work.
If I read everything correctly, what you describe doing is taking code owned by the first company and modifying it for the second company. That’s not at all a gray area. It’s a copyright violation. You the engineer sign away your rights to the code when you built it for company 1 while employed by them. Their employment contract for-sure states they own any work produced by you during your employment, and you agreed to this.
If the first project was done off of company time, posted publicly on a private account, you might have a claim to the rights.
I know you’ve dug your trench too deeply to change your mind at this point, but anyone reading your comments should know what you did was technically illegal and can get people in legal hot water.
I wrote the comment above, though I'm not the author of the code that you appear to think I am. But I am in agreement with him.
> Their employment contract for-sure states they own any work produced by you during your employment, and you agreed to this.
There are many open legal questions as to where this line is drawn. Surely the line falls somewhere between "every character I've ever typed on a keyboard" and "the verbatim code". I personally don't think he's crossed it. IP ownership is much more complex than portrayed in HBO's Silicon Valley. That is my opinion.
Furthermore, when I worked at GitHub (now acquired by Microsoft, so I'm sure things have changed drastically) -- there were very lax IP ownership agreements in the employment contracts around code ownership, because the legal department was worried that if found in any way conflicting with California law it would render the entire IP claims null and void (which does have precedent in California).
The point is we don't know, and I think OP would know better than us if it was disallowed or not.
Like many things in this area, the answer is usually "You'll find out if you want to go up against an army of lawyers". The last three companies I worked for all claimed ownership of any IP I create, on or off the job, using company's equipment or using my own equipment. One of them explicitly called it out during the interview: You will have to stop working on open source or publishing side projects when working here. Can they do that? Maybe, probably not. It doesn't matter because I do not plan to bankrupt myself fighting their lawyers.
Regardless of the fact that California is much, much more strict in what they allow, to the point where oftentimes a company’s lawyers won’t even try:
Fine. Don’t fight, I agree, that would be an unfair fight and a waste of time/money.
The US court system requires a “good faith” effort to settle the issue before it enters the legal system. A cease and desist for example— whatever it is, you’d have plenty of time to simply decide it’s not worth it and remove the code once they take notice.
IANAL: In California they cannot, unless it is related to the employer's business (so if the employer is Apple, Google or AWS, they probably can). In most states they can.
> and then later adapted and expanded upon that code (or even started over, with the knowledge of what you learned from others' work)
These are extremely different scenarios. Starting with a copyrighted material and modifying it is not at all the same as reading material and starting over. The first is violating copyright, the second is a derivative work.
If I read everything correctly, what you describe doing is taking code owned by the first company and modifying it for the second company. That’s not at all a gray area. It’s a copyright violation. You the engineer sign away your rights to the code when you built it for company 1 while employed by them. Their employment contract for-sure states they own any work produced by you during your employment, and you agreed to this.
If the first project was done off of company time, posted publicly on a private account, you might have a claim to the rights.
I know you’ve dug your trench too deeply to change your mind at this point, but anyone reading your comments should know what you did was technically illegal and can get people in legal hot water.