diff --git a/src/uix/eidos/active-eidos.svelte.ts b/src/uix/eidos/active-eidos.svelte.ts
index 7a7f54351..0301ccbed 100644
--- a/src/uix/eidos/active-eidos.svelte.ts
+++ b/src/uix/eidos/active-eidos.svelte.ts
@@ -42,6 +42,7 @@ import {
 	type RenderThemeCssOptions
 } from './lib/render-css';
 import { createThemeBaseEidosConfig } from './lib/themes/base';
+import { BUILTIN_JS_PRESETS } from './lib/motion/presets/js';
 import {
 	buildScheme,
 	schemeDeclarations,
@@ -326,6 +327,16 @@ export class ActiveEidos {
 			}
 		}
 
+		// JS-driven presets (the `spring` engine tier) are framework built-ins
+		// registered DIRECTLY: they carry functions (`MotionRun`) that the
+		// serializable, deep-cloned `EidosConfig` can't hold, so they don't ride
+		// `config.motion.presets`. soma's `Presence` runs them the same way.
+		if (this.#uix) {
+			for (const [name, preset] of Object.entries(BUILTIN_JS_PRESETS)) {
+				this.#uix.motion.register(name, preset);
+			}
+		}
+
 		if (this.#applyDom) {
 			this.apply();
 			this.#unsubscribe = this.#preferences.onPreferenceChange(() => this.apply());
diff --git a/src/uix/eidos/lib/motion/presets/js.ts b/src/uix/eidos/lib/motion/presets/js.ts
new file mode 100644
index 000000000..d88bbdb9f
--- /dev/null
+++ b/src/uix/eidos/lib/motion/presets/js.ts
@@ -0,0 +1,32 @@
+import { spring } from '$motion'
+import type { JsStatePreset } from '$motion'
+import { BUILTIN_CSS_PRESETS } from './css'
+
+/**
+ * Built-in JS-driven presets — the first consumers of the `arts/motion` engine's
+ * `spring` driver (until now built + tested but DORMANT: every shipped preset was
+ * `driver: 'css'`). Registered with `uix.motion` (active-eidos) exactly like the
+ * CSS presets, but RUN by the engine — semi-implicit-Euler physics stepped on
+ * `requestFrame`, the overshoot/settle curve no cubic-bezier expresses. soma's
+ * `Presence` calls `motion.run(node, phase)` and awaits `pending(el)`, which
+ * `getAnimations()` can't see (the spring is invisible to it).
+ *
+ * They generate NO CSS (`render-css` skips non-`css` drivers), so adding one needs
+ * NO `base.css` regeneration. Each carries a `fallback` CSS preset: with no DOM
+ * port (SSR / headless) or under reduced motion, the engine settles on the
+ * fallback instead of stepping the physics.
+ *
+ * Runs only on a Presence surface (an overlay): `<Popover.Content motion="spring-pop">`.
+ */
+export const BUILTIN_JS_PRESETS: Readonly<Record<string, JsStatePreset>> = {
+	// A PHYSICAL pop. Low damping on enter → it overshoots and bounces (a real
+	// spring, not a keyframe); the exit is stiffer (snappy, no bounce). Scales from
+	// the element's `transform-origin` (the floating origin on an anchored panel).
+	'spring-pop': {
+		driver: 'spring',
+		enter: spring({ values: { scale: [0.7, 1], opacity: [0, 1] }, stiffness: 300, damping: 14 }),
+		exit: spring({ values: { scale: [1, 0.86], opacity: [1, 0] }, stiffness: 360, damping: 30 }),
+		reduce: 'opacity-only',
+		fallback: BUILTIN_CSS_PRESETS['scale-fade']
+	}
+}
diff --git a/src/uix/eidos/lib/motion/registry.ts b/src/uix/eidos/lib/motion/registry.ts
index a2737564d..a90990cdf 100644
--- a/src/uix/eidos/lib/motion/registry.ts
+++ b/src/uix/eidos/lib/motion/registry.ts
@@ -29,6 +29,8 @@ export interface EidosMotionPresets {
 	'shared-axis-x': true
 	'shared-axis-y': true
 	'fade-through': true
+	// JS-driven (the `spring` engine tier) — runs via uix.motion, not CSS.
+	'spring-pop': true
 }
 
 /**
diff --git a/src/uix/eidos/lib/themes/base.ts b/src/uix/eidos/lib/themes/base.ts
index 2dc8dfc72..54b8149cc 100644
--- a/src/uix/eidos/lib/themes/base.ts
+++ b/src/uix/eidos/lib/themes/base.ts
@@ -495,6 +495,9 @@ export const THEME_BASE_OPTIONS: EidosConfig = defineEidosConfig({
 	motion: {
 		keyframes: BUILTIN_KEYFRAMES,
 		signatures: BUILTIN_SIGNATURES,
+		// CSS presets only — the EidosConfig is serializable (deep-cloned via
+		// `clonePlain`), and JS presets carry functions (`MotionRun`). The JS
+		// (`spring`) tier is registered DIRECTLY with the engine in `ActiveEidos`.
 		presets: BUILTIN_CSS_PRESETS
 	},
 	themes: {
diff --git a/src/uix/eidos/motion.test.ts b/src/uix/eidos/motion.test.ts
index 55dfc2420..2b8134387 100644
--- a/src/uix/eidos/motion.test.ts
+++ b/src/uix/eidos/motion.test.ts
@@ -3,6 +3,7 @@ import { describe, expect, it } from 'vitest';
 import { validateEidosConfig } from './lib/config';
 import { createEngineMotion } from '$motion';
 import { BUILTIN_CSS_PRESETS, BUILTIN_KEYFRAMES } from './lib/motion/presets/css';
+import { BUILTIN_JS_PRESETS } from './lib/motion/presets/js';
 import { renderStaticCss } from './lib/render-css';
 import { createThemeBaseEidosConfig } from './lib/themes/base';
 import type { EidosMotionPresets } from './lib/motion/registry';
@@ -51,8 +52,10 @@ describe('eidos motion — CSS generation', () => {
 		expect(css).toContain(`[data-animation-style='shared-axis-y'][data-state='open']`);
 		expect(css).toContain(`[data-animation-style='fade-through'][data-state='open']`);
 		expect(css).toContain('@keyframes scale-through');
-		// shared-axis composes a directional slide with a fade
-		expect(css).toMatch(/shared-axis-x'\]\[data-state='open'\] \{\s*animation: slide-axis-x-in/);
+		// shared-axis composes a directional slide with a fade. The enter selector is
+		// a comma-list (self-driven + the container-driven cascade branch), so match
+		// the rule's `animation:` past whatever selector precedes the brace.
+		expect(css).toMatch(/shared-axis-x'\][^{]*\{\s*animation: slide-axis-x-in/);
 	});
 
 	it('emits side-aware variants for slide-fade (data-side)', () => {
@@ -117,6 +120,17 @@ describe('eidos motion — runtime registry', () => {
 		expect(runtime.list()).toEqual(expect.arrayContaining(Object.keys(BUILTIN_CSS_PRESETS)));
 	});
 
+	it('registers the JS (spring) presets, resolving to their driver', () => {
+		// The engine's spring tier — registered DIRECTLY (not via the serializable
+		// config, which can't hold their `MotionRun` functions); `ActiveEidos` does
+		// this with `BUILTIN_JS_PRESETS`. Run by `uix.motion` (Presence), not the browser.
+		const engine = createEngineMotion();
+		for (const [name, preset] of Object.entries(BUILTIN_JS_PRESETS)) {
+			engine.register(name, preset);
+			expect(engine.resolve(name)?.driver).toBe('spring');
+		}
+	});
+
 	it('returns undefined for an unknown preset', () => {
 		expect(runtime.resolve('does-not-exist')).toBeUndefined();
 		expect(runtime.has('does-not-exist')).toBe(false);
@@ -236,11 +250,12 @@ describe('eidos motion — F6 token rigor', () => {
 });
 
 describe('eidos motion — F7 typegen registry', () => {
-	it('declares every built-in CSS preset as a type-safe registry name', () => {
+	it('declares every built-in preset (CSS + JS) as a type-safe registry name', () => {
 		// `registered` is the closed built-in set; the `satisfies` makes each a
 		// `keyof EidosMotionPresets` — a TYPO fails compile (the registry's
 		// `(string & {})` escape is NOT in play here). The runtime check pins it to
-		// the ACTUAL presets, so adding a built-in without registering it fails too.
+		// the ACTUAL presets (CSS + JS), so adding a built-in without registering it
+		// fails too.
 		const registered = [
 			'fade',
 			'scale-fade',
@@ -249,8 +264,11 @@ describe('eidos motion — F7 typegen registry', () => {
 			'collapse',
 			'shared-axis-x',
 			'shared-axis-y',
-			'fade-through'
+			'fade-through',
+			'spring-pop'
 		] as const satisfies readonly (keyof EidosMotionPresets)[];
-		expect([...Object.keys(BUILTIN_CSS_PRESETS)].sort()).toEqual([...registered].sort());
+		expect(
+			[...Object.keys(BUILTIN_CSS_PRESETS), ...Object.keys(BUILTIN_JS_PRESETS)].sort()
+		).toEqual([...registered].sort());
 	});
 });