Ok, how about "scrollable page" as the underlying mental model?

Drag and drop a scroll bar: now you have a scrollable page.

Do whatever you want to it.

People who never wrote fluid designs are always the first to tell you it's never been done.

It's still there in MacOS's XCode

When I've looked in the past, Interface Builder was (as far as I could tell) a first-step-and-don't-return thing. Meaning you could lay out an interface visually, but when you want to start coding it becomes a code definition of that interface, and once you start updating that, you can't (easily) return to the visual interface-definition interface. Has that changed, or was I wrong?

Sadly, that's what it has turned into.

Used to be a lot better.

I think "it's tricky" and "it exists" are compatible sentiments.