What about a situation when you develop something in a company, then company doesn't show interest in it and effectively shelve it, and then you leave the compa

What about a situation when you develop something in a company, then company doesn't show interest in it and effectively shelve it, and then you leave the company and want to come back to developing it? Be it in another company, which is yours or not? Technically it's a property of the previous company. However if you later enhanced and expanded it further, you ought to have a share. And in practice, don't we have some good examples of startups which were built on ideas initially rejected?