> Are you suggesting you would rather these UIs devote an unknown amount of screen real estate to various bulky OS scrollbars? That makes concise and predictable design for complex applications very difficult.

In all likelihood, I probably would. I chose my OS and one reason was those conventions. Especially on the web, developers seem to make very poor decisions on when to override things like scrolling. It's reasonable if, like you said, the UI is very complex and if I'm going to spend hours using it. In practice, they mess up conventions on a single scrolling page I'm spending 2 minutes on and it's usually a clue to avoid that site in the future.

> Those are the ones who need custom scrollbar solutions, and the way to get that done now is obtuse, brittle and hard to maintain.

That /has/ to be expected if you're stomping on the conventions. If you're changing the scrollbar, you're likely significantly changing the whole UI.

> It's reasonable if, like you said, the UI is very complex and if I'm going to spend hours using it. In practice, they mess up conventions on a single scrolling page I'm spending 2 minutes on and it's usually a clue to avoid that site in the future.

Scrolljacking and replacing the scrollbar are two very different things. And the problem is that there isn't a streamlined, consistent way to replace the scrollbar, so it's always a hacky mess.