I find `Surface`'s behaviour a little inconsistent from other containers, because it implicitly makes a

Box(propagateMinConstraints = true)

. I would expect the following to show a

Surface

with no color at all, because I haven't set any size for the

Spacer

inside it, but instead, the

Spacer

fills the entire

Surface

with its green color, as per the picture.

@Preview
@Composable
fun SurfaceBehaviour() {
    Surface(Modifier.padding(16.dp).fillMaxWidth().aspectRatio(16f / 9f)) {
        Spacer(Modifier.background(Color.Green))
    }
}

This doesn't really seem documented, either, so I'm curious as to why

Surface

would behave like this.

Surface behaves more like a decorator container for its child, the idea is that you can put it around your layout container instead of needing a SurfaceRow, SurfaceColumn, etc

Right. And I can maybe kinda see that, although it breaks my expectations of where modifiers are applied, still. And it also means that e.g. Card inherits the same functionality, which I definitely wouldn't expect.

Maybe it’s helpful to treat surface / card as a complex set of modifiers, in the same way you would add modifiers to a row or a column, you’re just decorating that layout with a surface / card.

I guess, but that's not obvious from the docs, IMO. In that case, would it make more sense to have a (public)

Modifier.surface

, for consistency? I'm mostly just asking because it made my own custom layout behave differently when it was wrapped in a Box vs. a Surface, which is something I wouldn't expect initially.

We investigated that a while ago, but a) there are some technical limitations because of the things that surface needs to handle (content color propagation for example), and also there is some benefit to having a semantic Surface / Card in the hierarchy - it’s a standalone concept that maps to Material guidance. Maybe worth filing a bug to expand the documentation for layout expectations a bit more here

Oh yeah, that makes sense. 👍 I can look into filing a request for expanding the docs a bit when I'm back at the office tomorrow. Thanks for explaining! 🙂