SwiftUI layout with GeometryReader inside ScrollView

I'm trying to create a scrollable grid of items. I create a custom view called GridView which uses GeometryReader to divide the space into columns and rows in HStacks and VStacks. But for some reason, its size shrinks to almost nothing when inside a ScrollView. In the screenshot you see the GridView (reddish) and it's parent VStack (greenish) have shrunk. The items inside the grid are still visible, rendered outside its area, but things do not scroll properly.

Why is the GridView not the size required to contain its items? If it did, I think this UI would scroll properly.

struct ContentView: View {
    var body: some View {
        ScrollView {
            VStack {
                Text("Section 1")
                GridView(columns: 2, items: [1, 3, 5, 7, 9, 11, 13, 15]) { num in
                    Text("Item \(num)")
                }
                .background(Color.red.opacity(0.2))
            }.background(Color.green.opacity(0.2))
        }.background(Color.blue.opacity(0.2))
    }
}

struct GridView<Content>: View where Content: View {
    var columns: Int
    let items: [Int]
    let content: (Int) -> Content

    init(columns: Int, items: [Int], @ViewBuilder content: @escaping (Int) -> Content) {
        self.columns = columns
        self.items = items
        self.content = content
    }
    var rowCount: Int {
        let (q, r) = items.count.quotientAndRemainder(dividingBy: columns)
        return q + (r == 0 ? 0 : 1)
    }
    func elementFor(_ r: Int, _ c: Int) -> Int? {
        let i = r * columns + c
        if i >= items.count { return nil }
        return items[i]
    }
    var body: some View {
        GeometryReader { geo in
            VStack {
                ForEach(0..<self.rowCount) { ri in
                   HStack {
                        ForEach(0..<self.columns) { ci in
                            Group {  
                                if self.elementFor(ri, ci) != nil {
                                    self.content(self.elementFor(ri, ci)!)
                                        .frame(width: geo.size.width / CGFloat(self.columns),
                                               height: geo.size.width / CGFloat(self.columns))
                                } else {
                                    Text("")
                                }
                            }
                        }
                    }
                }
            }
        }
    }
}