SwiftUI Layout & Containers

The layer between "stacks and spacers" and "it scrolls like butter with 100k rows" — Apple's Layout protocol, container composition, and the lazy-stack rules from the WWDC26 deep dive. View identity/data-flow questions route to swiftui/data-flow.

When This Skill Activates

• "Make these buttons equal width" / measurement-dependent layout
• Building a reusable container (custom List/board/carousel) that should accept ForEach + sections
• Lazy stack jank, memory growth, scroll-position bugs, broken scroll targeting
• Programmatic scrolling, paging/snapping, scroll-linked effects
• GeometryReader causing layout loops or mangled sizing

Custom Layout protocol (not GeometryReader)

Reach for a custom Layout whenever you must measure subviews and feed the measurement back into layout — GeometryReader only measures its container and can't influence the engine. Canonical case: equal-width buttons.