Multiple scrollable areas in a window is pretty annoying in my experience. Depending on what you last clicked on and where your cursor is can result in different areas being scrolled.
You're already buying into the whole app and there's good reason to not like the design decisions they made. I also think many companies overvalue the need for it.
Photoshop for most of its history used the OS' UI paradigms. Only in the past decade or so has it adopted its own. I'm pretty sure it used the OS' scroll bars at least until CS5 around 2010.
This "very, very narrow" number of applications you are suggesting is exactly what I was talking about. Those are the ones who need custom scrollbar solutions, and the way to get that done now is obtuse, brittle and hard to maintain.
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.
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.