Hacker News does everything it is supposed to according to the CSS standard to indicate to your browser that the area is scrollable. In fact, it is obvious that the site is doing enough because the browser does in fact render the area as scrollable. The fact that your browser doesn't do anything to visually indicate when a region is an independent scrollable area is entirely Apple's fault.

You can argue that websites should be written to conform to the standards-violating behavior in Apple's browser rather than actual Web standards, but we already went through that with Microsoft in the '90s and early 2000s, and I don't think you'll find a lot of people who are eager to return to those days. Browser-makers are expected to conform to the standard, not vice-versa.

I am sympathetic to the fact that this is inconvenient, because I use Apple's software too. But it's Apple that's responsible for the inconvenience, not all the web designers who are correctly following the standard.