And that rarely holds up in court, especially in California. Some common sense comes into play e.g. if you're building a direct competitor or using their IP, then of course that's going to be an issue.

> And that rarely holds up in court, especially in California.

Flowchart to help determine employer ownership of inventions: https://www.oncontracts.com/docs/Who-owns-an-employee-invent... (from 2010; annotated w/ citations to state statutes) (self-cite).

EDIT: Ownership of copyright in an employee-created "original work of authorship" will depend on the facts. Here are a couple of cases:

CASE 1: (A) The employee-author acted "within the scope of employment" in creating the work, AND (B) the employer and employee do not have an agreement altering ownership. RESULT 1: Under 17 U.S.C. 201(b), the employer owns the copyright in the work. https://www.law.cornell.edu/uscode/text/17/201

CASE 2: (A) Employee-author did not act "within the scope of employment" in creating the work, AND (B) the employer and employee did not have an agreement governing ownership of the work. RESULT 2: Under 17 U.S.C. 201(a), the employee owns the copyright in the work.

Note that calling something a "work made for hire" doesn't automatically make it one — the work must fit into one of nine statutory categories [0], AND the parties must agree, before the work is created, that the work will be a work made for hire.

[0] https://www.wikiwand.com/en/Community_for_Creative_Non-Viole...

Is "invention" to be interpreted in a very broad sense here? If I build some cookie cutter CRUD app that nevertheless provides an income, is that an invention?

It may not hold up in court, but your employer has more lawyers and more money than you and it simply may not be worth fighting.

That goes both ways. They know they'll lose. I've been in this exact situation with an estranged CEO. They send their cease & desist and hope you take it down, but won't go further than that. Sure, you might have a psycho on your hands ready to lose all his time and money, but at that point, you shouldn't have ever taken the job, so moot point.