@layer reset, tokens, base, layout, components, variants, states, utilities, overrides;

/* view-transitions — cross-document (MPA) page morphs
   (docs/components/view-transitions.md, T-89). OPT-IN ONLY: the build
   excludes this file from welkin.css — linking it IS the opt-in, because
   @view-transition changes the feel of every same-origin navigation and
   both the outgoing and incoming page must carry it. Firefox (View
   Transitions Level 1, no cross-document rules) ignores the unknown
   at-rule and navigates instantly; additive by nature, so no @supports
   gate exists or is needed (docs/03 contract row). */

/* Shared-element hook, following the tabs --wel-tabs-vt pattern: set
   --wel-vt to the same ident on one element of each page (--wel-vt: hero)
   and the pair morphs across the navigation. Registered non-inheriting —
   outside the motion gate, registration is inert — so a hero's name
   cannot cascade into its children: duplicate names abort the whole
   transition. */
@property --wel-vt {
  syntax: "*";
  inherits: false;
}

/* The --wel-motion multiplier cannot zero a UA-driven navigation
   animation, so the preference gates the whole feature off instead
   (docs/09: view-transition effects are gated, not just shortened). */
@media (prefers-reduced-motion: no-preference) {
  @view-transition {
    navigation: auto;
  }

  @layer components {
    * {
      view-transition-name: var(--wel-vt, none);
    }

    /* The universal rule above matches the root too and would strip the
       UA's `:root { view-transition-name: root }` — which is the whole
       default page cross-fade. Restore it (probe finding, T-89). */
    :root {
      view-transition-name: var(--wel-vt, root);
    }

    /* The reset's smooth scrolling animates cross-document FRAGMENT
       navigations (page.html#section) from the top of the new page —
       the transition captures at scroll 0 and then morphs against a
       page that is still scrolling thousands of pixels underneath
       (probed, T-89; history back/forward restoration is immune — it
       lands before capture). Instant scrolling while a transition is
       active puts the capture at the fragment. */
    :root:active-view-transition {
      scroll-behavior: auto;
    }

    /* Durations ride the motion tokens like everything else in Welkin;
       old/new stated too — engines don't inherit them from the group. */
    ::view-transition-group(*),
    ::view-transition-old(*),
    ::view-transition-new(*) {
      animation-duration: calc(var(--wel-motion-duration-3) * var(--wel-motion));
      animation-timing-function: var(--wel-motion-ease);
    }

    /* Image morph treatment (T-109): tag a --wel-vt-named image with
       data-vt-image on BOTH sides and its snapshots cover-fit the
       morphing group box instead of stretching between the two aspect
       ratios, and the pair swaps opaquely instead of the default
       cross-fade (a double-exposure flash on photographs). Works for the
       module's cross-document morphs and equally for an author's own
       same-document startViewTransition (a lightbox) — these pseudo
       rules are inert until some transition runs. view-transition-class
       is Level 2 (the spec's open question, graduated here): engines
       without it drop these two rules and keep the default morph —
       additive, no gate needed. */
    [data-vt-image] {
      view-transition-class: wel-image;
    }

    ::view-transition-old(*.wel-image),
    ::view-transition-new(*.wel-image) {
      block-size: 100%;
      inline-size: 100%;
      overflow: clip;
      object-fit: cover;
      mix-blend-mode: normal;
      /* no fade — the group's geometry morph is the whole effect; the
         new snapshot simply covers the old */
      animation: none;
    }
  }
}