>This is exactly why a hard, legally mandated and enforced working hours cap is needed
"Working hours" is a surprisingly nebulous concept. Let's say I'm a mechanic, and cars are my favorite thing in the world. The first thing I do after getting home from the shop is opening the hood of my personal project car, and tinkering with the engine for a few hours.
While my project car work doesn't strictly fall under "working hours," the skills I acquire in my home garage directly improve the quality of my work back at the shop. As a result, I will be a better mechanic than my colleagues who never touch a wrench outside of the shop, and I will thus get promoted faster than they do. Management notices this, and biases their hiring towards people who live, eat, and breathe cars because they make much better employees. Now, all mechanics at the shop are working 50+ hours a week on cars, even if by your hypothetical law they can only spend 35 of those hours at the shop.
Now let's make the definition of "working hours" even more nebulous. Suppose that a customer's car at the shop has a particularly interesting problem I couldn't solve during working hours. The problem is so interesting that I can't stop thinking about it when I get home. By law, I can't continue to work on the customer's car after hours, so instead I decide to reproduce the issue on my personal car, figure it out, and the next morning fix the customer's car as a result of my tinkering at home.
Replace "mechanic" with "software engineer," "customer's car" with "company codebase," and "personal car" with "personal computer," and you can further blur the line between working hours and personal hours.
The only law that could possibly prevent any of the above is making it illegal for people to work on anything even tangentially related to their day job during their spare time. This would not only be unenforceable, but patently unfair and ridiculous.
>>When a sizable fraction of researchers are in the lab 80 hours a week because they actually love their science that much
>Or they are in the lab for such long times because they are understaffed as hell, underfinanced, and under constant pressure to pump out "research" of any quality to chase tiny grants?
>No one except a very few select top researchers actually wants that life style that is incompatible with any form of social life.
That's exactly my point. As soon as you have even a few people in a workplace whose sole passion is the work, you're going to get a culture of overwork. Even if a scientist is legally mandated to only spend 35 hours a week in the lab, nothing’s stopping them from spending another 20 hours at home each week reading publications or working on other research-related tasks that can be done outside the lab (and for some scientists [e.g. computational scientists], that’s 100% of their tasks). I don't see any solution to this, other than strongly discouraging people who just want a 9-5 from working in fields that tend to attract people passionate about work. Just as not everyone is cut out for the grueling training schedule of a professional athlete (who submit themselves to such a regime because their sport is their passion), perhaps not everyone is cut out for the grueling work schedule of other professions.