Leave carousel items unmasked under reduced motion

Plan step 12, containment.md C-05. Only the parallax half of M3's rule
was honoured: the mask was still written every frame, so items kept
growing and shrinking between the keylines. Under reduced motion the
inset, the shift and the label fade are now all zero, so every item
stays at the strategy's large size and the keylines only decide where
the row snaps. `--material-carousel-pin` went with it: with no mask
there is nothing to pin the content to.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Qwx5USif3wFFmxtHg5U1g9

resources/js/carousel.js

@@ -23,9 +23,10 @@
  * records. A ResizeObserver does the same for a new width. RTL is read when measuring:
  * scroll offsets are negative there and shifts mirror, as Compose's `translationX` does.
  *
- * Under reduced motion the buttons and keys scroll instantly, and the content stays pinned to
+ * Under reduced motion the buttons and keys scroll instantly and nothing is masked at all: M3
- * the mask's leading edge instead of sliding inside it: the frame still opens and closes with
+ * says the parallax goes and items "should no longer expand as they come into view — all items
- * the scroll the user makes, but nothing moves on its own.
+ * are the same size", so every item stays at the strategy's large size and the keylines only
+ * decide where the row snaps.
  *
  * ---------------------------------------------------------------------------------------
  * Keyline maths ported from androidx (https://github.com/androidx/androidx), commit
@@ -872,7 +873,6 @@ document.addEventListener('alpine:init', () => {
                     .map((element) => ({
                         element,
                         surface: element.querySelector('[data-material-carousel-surface]'),
-                        content: element.querySelector('[data-material-carousel-content]'),
                         label: element.querySelector('[data-material-carousel-label]'),
                         labelWidth: 0,
                         written: '',
@@ -963,7 +963,13 @@ document.addEventListener('alpine:init', () => {
                 const scroll = this.scrollOffset()
                 const keylines = keylinesForScrollOffset(strategy, scroll, state.maxScroll)
                 const size = strategy.itemSize
-                const pinned = state.reducedMotion.matches
+
+                // M3: "When reduced motion settings are turned on, the parallax effect should be
+                // removed and carousel items should no longer expand as they come into view. All
+                // items are the same size" (docs/reference/m3/styles.md § Motion → Accessibility
+                // requirements). The keylines still decide where the row snaps; nothing is masked,
+                // moved or faded, so every item stays at strategy.itemSize.
+                const still = state.reducedMotion.matches
 
                 state.items.forEach((item, index) => {
                     const center = index * (size + strategy.itemSpacing) + size / 2 - scroll
@@ -978,11 +984,10 @@ document.addEventListener('alpine:init', () => {
                         translation += (center - keyline.unadjustedOffset) / keyline.size
                     }
 
-                    const inset = clamp((size - keyline.size) / 2, 0, size / 2)
+                    const inset = still ? 0 : clamp((size - keyline.size) / 2, 0, size / 2)
-                    const shift = state.rtl ? -translation : translation
+                    const shift = still ? 0 : state.rtl ? -translation : translation
-                    const pin = pinned ? (state.rtl ? -inset : inset) : 0
+                    const opacity = still || item.labelWidth === 0 ? 1 : clamp((item.labelWidth - size + keyline.size) / item.labelWidth, 0, 1)
-                    const opacity = item.labelWidth > 0 ? clamp((item.labelWidth - size + keyline.size) / item.labelWidth, 0, 1) : 1
+                    const written = `${inset.toFixed(2)}|${shift.toFixed(2)}|${opacity.toFixed(3)}`
-                    const written = `${inset.toFixed(2)}|${shift.toFixed(2)}|${pin.toFixed(2)}|${opacity.toFixed(3)}`
 
                     if (written === item.written) {
                         return
@@ -993,7 +998,6 @@ document.addEventListener('alpine:init', () => {
                     item.written = written
                     item.surface.style.setProperty('--material-carousel-inset', `${inset.toFixed(2)}px`)
                     item.surface.style.setProperty('--material-carousel-shift', `${shift.toFixed(2)}px`)
-                    item.surface.style.setProperty('--material-carousel-pin', `${pin.toFixed(2)}px`)
                     item.surface.style.setProperty('--material-carousel-label-shift', `${(state.rtl ? -inset : inset).toFixed(2)}px`)
                     item.surface.style.setProperty('--material-carousel-label', opacity.toFixed(3))
                 })

resources/views/components/carousel-item.blade.php

@@ -34,7 +34,7 @@
         data-material-carousel-surface
         class="relative size-full overflow-hidden rounded-corner-xl bg-surface-container-highest text-on-surface translate-x-(--material-carousel-shift) [clip-path:inset(0_var(--material-carousel-inset,0px)_round_var(--md-sys-shape-corner-xl))]"
     >
-        <div data-material-carousel-content class="size-full translate-x-(--material-carousel-pin) [&>img]:size-full [&>img]:object-cover">
+        <div data-material-carousel-content class="size-full [&>img]:size-full [&>img]:object-cover">
             {{ $slot }}
         </div>
 

resources/views/components/carousel.blade.php

@@ -40,8 +40,10 @@
      keys move one item, Home and End to the ends; focus moving into an item, or a press on one
      that is not fully open, brings it into focus. `controls` adds previous and next icon
      buttons under the row — by default only where the pointer is fine (a mouse or trackpad);
-     `true` always, `false` never. Under reduced motion they scroll instantly and content no
+     `true` always, `false` never. Under reduced motion they scroll instantly and nothing is
-     longer slides inside its mask. RTL mirrors the keylines, keys and buttons.
+     masked: every item stays at its large size, which is M3's rule for a carousel under reduced
+     motion (docs/reference/m3/styles.md § Motion → Accessibility requirements). RTL mirrors the
+     keylines, keys and buttons.
 
      Re-measures itself when resized, when a Livewire morph resets its styles and when items
      come and go. The row's id, which the buttons control, is new with every render; the row

tests/Browser/CarouselTest.php

@@ -146,15 +146,14 @@ it('brings a partly hidden item into focus when it is pressed', function () {
     $page->assertScript(onCarousel(0, 'return inset(4) < 0.5 && scroller.scrollLeft > 0'));
 });
 
-it('scrolls instantly and keeps content pinned to its mask under reduced motion', function () {
+it('scrolls instantly and leaves every item unmasked under reduced motion', function () {
     carouselShowcase(['reducedMotion' => 'reduce'])
         ->assertScript(onCarousel(0, <<<'JS'
             root.querySelector('[aria-label="Next"]').click()
             const arrived = at(1)
             await pause(50)
-            const last = items.length - 1
+            return arrived && items.every((_, i) => inset(i) === 0)
-            return arrived && inset(last) > 0.5
+                && items.every((item) => Math.abs(item.getBoundingClientRect().width - size) < 1.5)
-                && surface(last).style.getPropertyValue('--material-carousel-pin') === surface(last).style.getPropertyValue('--material-carousel-inset')
             JS));
 });