Liquid Glass — motion, animation, and shaders

Companion to the liquid-glass skill (~/.claude/skills/liquid-glass/). Same tag system: [verified] / [apple] / [multi] / [single] / [version-pinned] — see that skill's SKILL.md. Composed 2026-08 and verified against Apple documentation and SDK interfaces.

Operating rules

1. Before generating code, ALWAYS read the availability firewall in the companion liquid-glass skill (installed alongside this one): references/availability-and-sdk27.md. Motion code trips the same SDK-27 breakages (.if ban, @ContentBuilder, renames).
2. Load references on demand via the router — one topic, one file.
3. The motion model: Liquid Glass and its motion were designed together. Glass does not fade — it materializes by modulating lensing; when glass grows, its material gets thicker (deeper shadow, stronger lensing). Size change is a material change, which is why morphs must be the system animation, never hand-faked.
4. Verify motion claims against references/ before repeating them — circulating write-ups get several of these facts wrong; the corrected facts live in these files.
5. Profiling evidence beats reasoning: record an Instruments trace with xctrace before optimizing, and never trust the iOS Simulator for the SwiftUI instrument — see references/performance-and-instrumentation.md.

Hard rules (the short list)

1. Never animate glass with .opacity / fades — use glassEffectTransition (.materialize is the sanctioned fade-like option). [multi]
2. Morphing silently no-ops unless ALL four hold: same GlassEffectContainer; glassEffectID per element in one shared @Namespace; the change is view insertion/removal (not a value change); the state change is inside withAnimation. Plus geometry: nearest edges within the container's spacing. [multi]
3. The interactive "squish" is one boolean — .interactive(). No public spring/damping/ stiffness knobs exist; do not promise tunable jelly. [multi]
4. Gate every glass animation on accessibilityReduceMotion — the default morph is a kinetic effect, and reduce-motion users have experienced it as flickering. The system does NOT gate your withAnimation for you. [multi]
5. One animation source per property — never mix implicit .animation, withAnimation, and scroll-driven updates on the same property. Implicit later in the view tree silently wins over explicit. [multi]
6. The animation context must live OUTSIDE a conditional — an .animation() inside the if is destroyed with the view and removal never animates. [multi]
7. Prefer renderer-side motion for scroll: scrollTransition / visualEffect(in:) over observable scroll-offset plumbing. [apple]
8. Animate transforms first (scale/offset/rotation), frames second, identity changes last. [multi]

Topic router

Topic	Reference
Springs, curves, precedence, Transaction, @Animatable, CA bridge	references/animation-fundamentals.md
Glass morphing, glassEffectID, union, glassEffectTransition	references/morphing.md
Navigation zoom, custom Transition, sheets, touch-vs-trackpad	references/transitions-and-navigation.md
Scroll-driven motion, scrollTransition, visualEffect, chrome interplay	references/scroll-driven.md
PhaseAnimator, KeyframeAnimator, symbol effects	references/phase-and-keyframe.md
Reduce Motion gating, replacement tables	references/reduce-motion.md
Metal shaders with/under/instead-of glass	references/metal-and-glass.md
Performance model, xctrace profiling	references/performance-and-instrumentation.md
Static glass, availability, toolbars, platform APIs, a11y of the material	liquid-glass skill
Choosing the actual spring/damping/duration values	apple-motion-feel skill

Scripts

scripts/record_trace.py and scripts/analyze_trace.py (+ instruments_parser/) — vendored from Antoine van der Lee's SwiftUI-Agent-Skill (MIT; licence ships beside them as scripts/LICENSE-AvdLee-SwiftUI-Agent-Skill). Record and parse Instruments traces (Time Profiler, Hangs, Animation Hitches, SwiftUI lanes) into JSON/markdown. Usage and interpretation: references/performance-and-instrumentation.md.