---
name: theme-crossfade-morph
description: Whole-theme in-place morph under a fixed anchor — background, typography, corner radii, icons, chrome and logos all blend simultaneously (~0.3s) through N pre-styled skins while one anchor element never moves. Recipe = stacked full layers + opacity crossfade, anchor rendered once on top. Seek-safe by construction.
metadata:
  tags: theme, skin, crossfade, morph, anchor, reskin, cycle, ui, stacked-layers
---

# Theme Crossfade Morph

The whole world re-skins while one thing holds still. A composer box cycles through four IDE themes; a checkout widget flips through brand skins — background, typography, corner radii, toolbar icons, footer logos all change **at once**, in place, in ~0.3s, N times — and through every flip one anchor element (the prompt string, the widget layout, the wordmark) **never moves**. The anchor's stillness is the rhetorical claim: _everything changes, this doesn't._

Boundary: [card-morph-anchor.md](card-morph-anchor.md) morphs **one container** between two shots — its dimensions, radius, and surface tween continuously. This rule re-skins an **entire scene** through **N discrete states**: nothing tweens property-by-property (fonts, icons, and logos can't interpolate); the "morph" is a fast simultaneous crossfade of complete pre-styled layers. ([scale-swap-transition.md](scale-swap-transition.md) swaps an element at center; here the surroundings swap and the element holds.)

## How It Works

1. **One skin = one complete layer.** Each theme state is a fully pre-styled, full-bleed layer (`position: absolute; inset: 0`) containing everything that changes: background, shell/chrome, toolbar icons, footer logos, typography. All `N_SKINS` layers exist in the DOM from `t=0`, stacked; skin 0 starts visible, the rest at `opacity: 0`.
2. **The morph is a crossfade.** At each boundary, two opposing opacity tweens run at the same timeline position over `MORPH_DUR` (~0.3s): outgoing `1 → 0`, incoming `0 → 1`. Because both layers are complete, every property "blends" simultaneously for free — including the un-tweenable ones (font families, icon glyphs, logos), which read as morphing precisely because everything else is mid-blend around them.
3. **The anchor renders once, on top.** The element that must not move lives in its own layer above all skins and is **excluded from every skin layer**. No transforms, no re-parenting, no per-skin restyle.
4. **Windows are precomputed.** `T_k = CYCLE_START + k × (SKIN_HOLD + MORPH_DUR)`. Steady cadence by default; hold the final skin longest when it's the resolve.

The only animated property is `opacity` — which is why this rule is seek-safe with zero special machinery.

## Recipe

```html
<!-- inside a standard scene clip (hyperframes-core) -->
<div class="theme-stage">
  <!-- One complete pre-styled layer per skin; skin-0 visible at t=0 -->
  <div class="skin skin-0"><div class="shell">…terminal chrome, mono type, footer badge…</div></div>
  <div class="skin skin-1">
    <div class="shell">…rounded composer, sans type, toolbar pills, logo…</div>
  </div>
  <div class="skin skin-2"><div class="shell">…dark shell, its own chrome and footer…</div></div>

  <!-- The anchor: rendered ONCE, above every skin. It never moves. -->
  <div class="anchor" id="anchor">{anchorText}</div>
</div>
```

```css
.theme-stage {
  position: absolute;
  inset: 0;
}
.skin {
  position: absolute;
  inset: 0;
  opacity: 0;
  /* Each skin fully self-styled: its own background, fonts, radii,
     icons, chrome, logos. Nothing inherited across skins. */
}
.skin-0 {
  opacity: 1; /* the opening state — matches the timeline's fromTo */
}
.shell {
  /* CRITICAL: shared geometry. The shell box (and any element that
     "persists" across skins — toolbar row, footer row) sits at the SAME
     coordinates in every skin, so mid-blend frames read as one UI
     changing clothes, not two UIs ghosting. */
  position: absolute;
  left: SHELL_LEFT;
  top: SHELL_TOP;
  width: SHELL_WIDTH;
  height: SHELL_HEIGHT;
}
.anchor {
  position: absolute;
  z-index: 10; /* above every skin */
  left: ANCHOR_LEFT;
  top: ANCHOR_TOP;
  /* No transforms, no transitions — the stillness is load-bearing. */
}
```

```js
const skins = gsap.utils.toArray(".skin");

// Boundary k→k+1 at T_k: outgoing fades down as incoming fades up —
// ONE simultaneous crossfade, everything blends at once.
skins.forEach((skin, k) => {
  if (k === 0) return; // skin-0 is the opening state
  const at = CYCLE_START + k * (SKIN_HOLD + MORPH_DUR);
  tl.fromTo(skin, { opacity: 0 }, { opacity: 1, duration: MORPH_DUR, ease: "power2.inOut" }, at);
  tl.to(
    skins[k - 1],
    { opacity: 0, duration: MORPH_DUR, ease: "power2.inOut" },
    at, // same position — the blend is simultaneous, never sequential
  );
});

// The anchor gets NO tweens. Its absence from the timeline is the point.
```

## Variations

- **Anchor-typography reskin (per-layer copies)** — when the anchor's own type treatment must change with the theme (mono in the terminal skin, sans in the editor skin), each skin carries its own copy of the anchor at **pixel-identical geometry** and there is no separate top layer; the invariant shifts from "one element" to "one geometry." Verify the copies overlay exactly (screenshot two skins at 50% opacity) — a 2px baseline drift reads as the anchor flinching, which breaks the whole claim.
- **Skin-cycle tour with logo relay** — a large brand logo outside the anchored shell crossfades **in the same windows** as the skins (logo k with skin k, same `MORPH_DUR`). The paired swap sells "same product, every brand."
- **Washout finale** — after the last skin, a final low-key layer (faint dot-grid, blueprint wash) fades in while the last shell drops to ~0.25 opacity — the cycle resolves into a held diagram of itself. One extra window; the anchor may fade with the shell or hold full-strength.
- **Emphasis brake** — steady cadence for `N−1` skins, then hold the final skin 2–3× `SKIN_HOLD`; the cycle demonstrates breadth, the brake lands the resolve. Precompute the hold array; don't drift the cadence without cause.

## Values

| token           | range                                       | notes                                                                                                |
| --------------- | ------------------------------------------- | ---------------------------------------------------------------------------------------------------- |
| N_SKINS         | 3–5                                         | two is a before/after (consider `card-morph-anchor`); past five the cycle pads                       |
| SKIN_HOLD       | 0.8–1.5s                                    | long enough to register the logo/footer identity, short enough to keep the churn rhetorical          |
| MORPH_DUR       | 0.25–0.4s, ~0.3s canonical                  | faster reads as a hard cut; slower reads as a mushy dissolve with lingering double-exposure          |
| CYCLE_START     | ≥ anchor settle + a beat                    | after the anchor and skin-0 have fully registered                                                    |
| SHELL geometry  | —                                           | shell / toolbar / footer coordinates identical across skins; contents inside the slots differ freely |
| ANCHOR position | —                                           | identical to the pixel across the scene (per-layer form: identical in every skin)                    |
| washout / brake | shell ~0.2–0.3 opacity; hold 2–3× SKIN_HOLD | —                                                                                                    |

## Critical Constraints

- **The anchor never moves.** No transforms, no opacity dips, no re-parenting, no restyle — the contrast between total churn and total stillness is the entire device; one flinch and the shot becomes a slideshow.
- **Nothing tweens but `opacity`** — no `borderRadius` / `background` tweens; radii and colors change by being different in the next layer. Visibility via `opacity` only, never `display` / `visibility` toggles (they can't blend mid-fade).
- **Pixel-align the shared geometry** — mid-blend both skins are partially visible; aligned shells read as one UI changing clothes, misaligned shells ghost into two UIs.
- **Pre-style everything** — each skin is complete and static; no class toggling, no runtime restyle mid-tween.
- **Outgoing and incoming tweens share one timeline position** — a staggered blend flashes the stage background between skins.
- **Adjacent windows only** — skin k crossfades with k+1, never k+2; at no frame are three skins partially visible.
- **Camera static — always.** A push-in on top of a theme cycle destroys the stillness that makes the anchor read.
- **Hard cuts are the cheaper sibling** — if the states should _snap_, that's `discrete-text-sequence` territory; the ~0.3s blend is specifically the "morph" read.

## See also

`context-sensitive-cursor` (caret color switches at each `T_k`) · `discrete-text-sequence` (type the anchor first; or the hard-cut alternative) · `card-morph-anchor` (the single-container sibling) · `spring-pop-entrance` (the lockup that joins the anchor at the resolve) · `sine-wave-loop` (drifting field under the cycle — never on the anchor).
