Bad news. I knew this day would come eventually, the day when a browser change would "break" Highslide JS, and since it's abandonware, it would never be fixed. Remember, the last substantive update to this script dates back over 8 years, and even that was just a minor tweak.
It turns out that even Roadrash's old demo page doesn't work properly in Safari. I put together a very "clean" test of this kind of iframe expander, using only HTML 5, with the same result. In all three cases (Roadrash's page, your page, my page), the scrolling worked only if I first "poked" the expander, either by shrinking or enlarging it (using the little resizer at lower right), or if I dragged the window by the header, even just a short distance. After that, lo and behold, scrolling works again.
I'll keep tinkering with it, just to see if there's one particular parameter that's kicking it in the butt, but I'm not hopeful.
This is why I stopped using Highslide JS a couple of years ago, and started using Floatbox, which is actively maintained. But that's not a simple "swap-out, swap-in" replacement - the page coding is completely different.