Personally, at the same time as signing the employment contract, I would create a list of projects I develop/contribute to outside of work with a description and links, and have them sign an acknowledgement these don't interfere with their "business" (and if they refuse to sign simply resign on the spot the same second), whether or not they use them already for business purposes or not, and an obligation for them to notify me if they ever start using one of my projects internally in the future.

Also, for open source projects I contribute too that the company uses I'd have a separate paper stating my role in them, and both of our expectation for contributions that are made during/outside of hours, again signed by both parties.

For FOSS projects that I own that the company wants to use, I'll have them clearly define my scope in them during employment, have them assign copyright ownership to me for any contribution they want upstreamed, and have every commit with their hashes reviewed and approved by a lead/manager.

But as far as I'm concerned, there is no point in doing any of this if the employment contract states they own all my IP, since the documents would contradict one another. That's why cleaning up the mess in the employment contract is such a big deal for me.