Videos start at where I left them last time, or beginning if never watched. This worked since forever (unless you disable history completely). I believe they store it in localStorage.

Edit: after second thought, maybe they store it on server too. Anyway, everything worked almost perfectly. I could lose a timestamp once a week, but probably due to some packet error.

Someone at YT literally broke what worked, in a way that is so naive and non-web2.0, as if it was made by a php junior from early 2000s. Because e.g. if you open history and newtab a link with &t=, then you have to refresh history now after watching few minutes of the video. And every click makes it worse cause it overwrites internal data too. The original “internal” method worked perfectly because it never relied on a url that may and will become obsolete. Idk why they did that and how it was approved etc.

Super interesting. Thanks for the details! (I don't work there or anything, was just curious about the implementation)