An alternative to ViewThatFits. Updated version can be found here: https://github.com/ryanlintott/LayoutThatFits

LayoutThatFits.swift

//
//  LayoutThatFits.swift
//  WWDC22Experiments
//
//  Created by Ryan Lintott on 2022-06-08.
//

import SwiftUI

struct LayoutThatFits: Layout {
    let axes: Axis.Set
    let layoutPreferences: [any Layout]
    
    init(in axes: Axis.Set = [.horizontal, .vertical], _ layoutPreferences: [any Layout]) {
        self.axes = axes
        self.layoutPreferences = layoutPreferences
    }
    
    var layouts: [AnyLayout] {
        layoutPreferences.map { AnyLayout($0) }
    }
    
    func layoutThatFits(proposal: ProposedViewSize, subviews: Subviews, cache: inout ()) -> AnyLayout? {
        layouts.first(where: { layout in
            var cache = layout.makeCache(subviews: subviews)
            let size = layout.sizeThatFits(proposal: proposal, subviews: subviews, cache: &cache)
            
            let widthFits = size.width <= (proposal.width ?? .infinity)
            let heightFits = size.height <= (proposal.height ?? .infinity)
            
            return (widthFits || !axes.contains(.horizontal)) && (heightFits || !axes.contains(.vertical))
        })
    }
    
    func sizeThatFits(proposal: ProposedViewSize, subviews: Subviews, cache: inout ()) -> CGSize {
        let layout = layoutThatFits(proposal: proposal, subviews: subviews, cache: &cache) ?? layouts.last!
        var cache = layout.makeCache(subviews: subviews)
        return layout.sizeThatFits(proposal: proposal, subviews: subviews, cache: &cache)
    }
    
    func placeSubviews(in bounds: CGRect, proposal: ProposedViewSize, subviews: Subviews, cache: inout ()) {
        let layout = layoutThatFits(proposal: proposal, subviews: subviews, cache: &cache) ?? layouts.last!
        var cache = layout.makeCache(subviews: subviews)
        layout.placeSubviews(in: bounds, proposal: proposal, subviews: subviews, cache: &cache)
    }
}

struct ViewThatFitsExample: View {
    @State private var width: CGFloat = 300
    
    var content: some View {
        Group {
            Text("View")
            Text("That")
            Text("Fits")
        }
        .lineLimit(1)
        .padding()
        .foregroundColor(.white)
        .background(Color.blue)
        .fixedSize(horizontal: true, vertical: false)
    }
    
    var body: some View {
        VStack {
            Slider(value: $width, in: 50...400)
            
            Text("ViewThatFits").font(.title)
            Text("- Duplicate sets of subviews")
            Text("- Animation not possible")
            
            Text(
"""
ViewThatFits {
    HStack {
        content
    }
    VStack {
        content
    }
}
"""
            )
            .font(.body.monospaced())
            .padding()
            
            
            Spacer()

            ViewThatFits {
                HStack {
                    content
                }
                VStack {
                    content
                }
            }
            .frame(width: width)
            .background(Color.gray)
            .animation(.default, value: width)

            Spacer()
        }
        .padding()
    }
}

struct LayoutThatFitsExample: View {
    @State private var width: CGFloat = 300
    
    var content: some View {
        Group {
            Text("Layout")
            Text("That")
            Text("Fits")
        }
        .lineLimit(1)
        .padding()
        .foregroundColor(.white)
        .background(Color.blue)
        .fixedSize(horizontal: true, vertical: false)
    }
    
    var body: some View {
        VStack {
            Slider(value: $width, in: 50...400)
            
            Text("LayoutThatFits").font(.title)
            Text("- One set of subviews")
            Text("- Animation works!")
            
            Text(
"""
LayoutThatFits(
    in: [.horizontal],
    [HStack(), VStack()]
) {
    content
}
"""
            )
            .font(.body.monospaced())
            .padding()
            
            Spacer()

            LayoutThatFits(in: [.horizontal], [HStack(), VStack()]) {
                content
            }
            .frame(width: width)
            .background(Color.gray)
            .animation(.default, value: width)

            Spacer()
        }
        .padding()
    }
}

struct LayoutThatFits_Previews: PreviewProvider {
    static var previews: some View {
        ViewThatFitsExample()
        LayoutThatFitsExample()
    }
}

Related to this twitter thread: https://twitter.com/ryanlintott/status/1534706352177700871