Also, why root surface is expanded on a whole screen while surface enclosing say an EditableText does not?

Surface is taking a size of it's children. there are some layout which are taking all the available space and some that wraps. For example there is a Wrap component for this

hmm - this is kinda confusing though, isn’t it? You’re saying that

Surface { }

ends up expanding to fill all space, but

Surface { Text(...) }

will wrap the text?

Surface with no child will use min constraint size, which should be 0. but a lot Layouts we have take all the available space by default. So if you wrap them into Surface that would fill the space