Pass sibling to Renderer.mount, fix update order

[MaxDesiatov]

Resolves, but adds no test cases to the test suite for #294. See the issue for the detailed description of the problem.

I will add end-to-end tests for this in future PRs.

I've tested these cases manually so far:

struct Choice: View {
  @State private var choice = false

  var body: some View {
    HStack {
      Button("Trigger") {
        choice.toggle()
      }
      if choice {
        Group {
          Text("true")
          Text("true")
        }
      } else {
        VStack {
          Text("false")
        }
      }
      Text("end")
    }
  }
}

Note the Group view with multiple children in this one, it uncovered required checks for GroupView conformance.

Also tested these more simple cases:

struct Choice: View {
  @State private var choice = false

  var body: some View {
    HStack {
      Button("Trigger") {
        choice.toggle()
      }
      if choice {
        Group {
          // single child
          Text("true")
        }
      } else {
        VStack {
          Text("false")
        }
      }
      Text("end")
    }
  }
}

and

struct Choice: View {
  @State private var choice = false

  var body: some View {
    HStack {
      Button("Trigger") {
        choice.toggle()
      }
      if choice {
        // single child, no nesting
        Text("true")
      } else {
        VStack {
          Text("false")
        }
      }
      Text("end")
    }
  }
}

[MaxDesiatov]

It wasn't referenced anywhere, so no need to have it as a requirement. It also allowed making this property completely private in Renderer implementations.

[MaxDesiatov]

Still keeping the property implementation itself around, as we need a non-zero retain count on it. The reconciler is really what drives the rendering process.

[carson-katri]

Looks good! The original issue was a great catch 😅