If I'm someplace other than where I want to be, it's work.
A lot of comments in this thread don't seem to understand - even if you're free to read HN during downtime, if I need to be in by 9am and am unable to leave until 7pm, the period in between was work. If I would have suffered professionally if I'd left at 5 regardless of how much "work" was done, the performative behavior is also "work".
Labor law in the states uses "engaged to work" and similar language often, mostly in the context of hourly employees. You can't tell someone "you must show up to the jobsite and wait to see if you're needed" under threat of no future work... that's being engaged to work.