Making two views match sizes in SwiftUI is surprisingly simple: give each an infinite max frame in the direction you want, then apply fixedSize() to the container. No GeometryReader required. See how it works: https://www.hackingwithswift.com/quick-start/swiftui/how-to-make-two-views-the-same-width-or-height
How to make two views the same width or height - a free SwiftUI by Example tutorial

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@twostraws _Actually_ this makes total sense if you know that SwiftUI containers offer a size proposal to their subviews which they refine and communicate back up to the containers which then loops back around for a final pass before deciding the actual sizes. Have you never even _tried_ reading the docs for each individual method requirement of the Layout protocol? 🧐

Seriously though, sometimes I miss constraints.