As of last time I checked a few years ago, NY law was silent on rules for the copyright ownership scenario you describe.
That leaves the federal default in place of "creator keeps the copyright when there is no explicit assignment unless it qualifies as a work for hire." Only work done within the scope of employment would be employer-owned by default, not unrelated spare time projects.
However, indeed NY's legal silence on this point allows contracts to explicitly assign the category of work you describe to the employer, if they include wording to that effect, subject only to any case law that might seem something excessive (not sure what NY courts have said on this topic).
The difference with California in this regard is that they actually prohibit and refuse to enforce contractual assignment of unrelated spare-time projects to the employer. Far stronger than a default.
The attitude to post-employment non-competes is also very different: aside from a very narrow M&A exception, California flat out bans those. NY disfavors them and courts often limit or discard them as unreasonable upon examination, but they're not categorically banned. That has a significant chilling effect.
Note I'm not a lawyer, just a law-geek former law student layman. Pay for qualified legal advice with appropriate licensure if you need that (I do that too despite my legal interests).
As of 2002, the advice of a lawyer specializing in intellectual property law to me was that work created by a professional employee on their own time is by default a work for hire, and I was a professional employee. I later met other people who were in the same situation.
Telling whether I was a professional employee was simple. The three categories are contractor or hourly. If you don't know yourself to be a contractor, and you don't punch a clock, then you are a professional employee. Most software developers in New York are either contractors or professional employees.
New York law could have changed since then. This is not legal advice, and I am not a lawyer. However it is my understanding of legal advice that I actually received.
As you note, California law is much stronger. The one catch is that your employer can claim ownership of anything related to what they do. Not what you do for them, but what they do. If your employer has their fingers in a lot of pies, like Google and Amazon do, you may not realize that what you're doing is related to something that they do.
I can believe that work done by professional employees outside of work hours related to their employment might sometimes be works for hire, since they're not hiring professional employees. But that just covers work related to the employment, not everything the employer might do.
If you're an accountant for Bloomberg NY who somehow avoided any contract provisions affecting this question (itself an unlikely scenario), they wouldn't own any spare-time Kubernetes hacking you do, since that's not part of an accountant's job.
In California, a contract might be able to make the employer own the Bloomberg accountant's spare time Kubernetes hacking only if the courts decide that it's related to other parts of Bloomberg's business, which is hard to predict; indeed, parts of Bloomberg do deploy and hack on Kubernetes and use it to run important stuff for their business, but of course their main business line is financial news and not container tech.
The difference with New York is that sufficiently inclusive contract wording would be able to assign that to the employer even if Kubernetes were not related in any way to Bloomberg's business, unless NY courts would it unreasonable to apply that wording.
(For example, I doubt NY courts would find it reasonable to assign any employers copyright over an employee's written grocery shopping list, aside from household nanny/errand-runner situations.)
The work-for-hire rules are actually federal: anything covered by them is employer-owned by default in all of the US including California, not dependent on a contractual provision which California could choose not to enforce, with the employer being treated as the actual author for copyright purposes rather than just an assignee.
So the only relevant difference between the states would apply in cases where federal law doesn't automatically do that, where people incorrectly assume it doesn't do that, or where a contract or other agreement by the employer explicitly overrides the work-for-hire rule (if this is legal) or reassigns or disclaims the employer's interest in certain created works.
I'd be interested in a citation to either a court ruling, a statute, a regulation, or similar to clarify this as applied to software workers.
You'll have to talk to a proper lawyer to get a citation.
The state difference emerges because the "for hire" bit depends on local labor laws. And yes, the type of work matters. If a programmer writes a novel, or an accountant writes software, the employer likely doesn't have a case. But if a programmer writes software, then in New York it doesn't matter whether the software relates to your job or what your employer does. In California the content does matter.
Well, there are two different things here: for-hire status or not, and assignment to employer if not.
For-hire status is different since the employing corporation is actually deemed the author, which affects things like duration of copyright. Maybe it depends on whether a state-law employment relationship was in place or what the state says your job functions are. I'm unsure about that point but I don't think either of us is claiming meaningful differences between California and New York on that point.
In either state, if you do something within the scope of your employment (i.e. programmers writing software for their employer's use or in-house accountants writing notes to a financial report for their employer), the employer owns it as author regardless of contract, unless the employer explicitly assigns it in writing to the employee.
Also in both states, nothing the employee does in their spare time outside of the scope of employment is owned by the employer by default, in the absence of a contract saying otherwise.
The only difference in this regard between California and New York is that a California statute restricts what the contract can assign. New York has no such statute.
But your example about a programmer writing a novel is interesting: if the content of the novel relates to the actual or demonstrably anticipated business or research of the employer, or uses the employer's confidential information / resources / time, an assignment provision covering it would be enforced in California. Same if an accountant writes software useful for the Kubernetes hackers at their employer.
Neither of these examples would be a work for hire or assigned by default, but both California and New York would allow that to be assigned by employment contract language (with the individual and not the corporation as author for copyright law purposes).
The relevant difference as I understand it is the definition of "the scope of the employment".
In both states, if a professional employee figured out how to solve a work problem at 6 AM in the shower, that is clearly part of the scope of the employment. The difference is that if a programmer sits down to write some code at 8 PM, that is more likely to fall within the scope of employment in New York than in California.
This difference is on top of what the contract is allowed to say.
That leaves the federal default in place of "creator keeps the copyright when there is no explicit assignment unless it qualifies as a work for hire." Only work done within the scope of employment would be employer-owned by default, not unrelated spare time projects.
However, indeed NY's legal silence on this point allows contracts to explicitly assign the category of work you describe to the employer, if they include wording to that effect, subject only to any case law that might seem something excessive (not sure what NY courts have said on this topic).
The difference with California in this regard is that they actually prohibit and refuse to enforce contractual assignment of unrelated spare-time projects to the employer. Far stronger than a default.
The attitude to post-employment non-competes is also very different: aside from a very narrow M&A exception, California flat out bans those. NY disfavors them and courts often limit or discard them as unreasonable upon examination, but they're not categorically banned. That has a significant chilling effect.
Note I'm not a lawyer, just a law-geek former law student layman. Pay for qualified legal advice with appropriate licensure if you need that (I do that too despite my legal interests).