Prove again that the carousel scrolls instantly under reduced motion

Plan step 38 review: de-flaking the reduced-motion test moved the click
out of the retried assertScript, which was right, but also dropped the
arrival read in the click's own task, so a smooth scroll finishing
within assertScript's retries would now pass a test named "scrolls
instantly". The click script records at(1) at once and the assertion
requires it. It is read against the slot size measured before the
click: the row re-measures a frame later (222 against 217.7px at
1280px), which is why a later read in the same evaluation fails.
Chromium: CarouselTest 10 of 10, the test itself three runs green.

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

tests/Browser/CarouselTest.php

@@ -170,14 +170,18 @@ it('brings a partly hidden item into focus when it is pressed, and leaves an ope
 it('scrolls instantly and leaves every item unmasked under reduced motion', function () {
     // A click, then a separate read-only assertScript: assertScript retries its whole expression
     // on a false result (this file's own comment above), and a click inside it would fire again on
-    // every retry, drifting the carousel one further item each time and never recovering.
+    // every retry, drifting the carousel one further item each time and never recovering. Whether
+    // the row had arrived is read in the same task as the click, against the slot size measured
+    // before it, and kept, so a retry cannot turn a smooth scroll that finishes later into a pass:
+    // under reduced motion it must be instant. (The row re-measures its slot a frame later, so the
+    // assertion below reads a fresh size rather than this one.)
     $page = carouselShowcase(['reducedMotion' => 'reduce']);
 
-    $page->script(onCarousel(0, 'root.querySelector(\'[aria-label="Next"]\').click()'));
+    $page->script(onCarousel(0, 'root.querySelector(\'[aria-label="Next"]\').click(); window.__arrivedAtOnce = at(1)'));
 
     $page->assertScript(onCarousel(0, <<<'JS'
         await pause(50)
-        return at(1) && items.every((_, i) => inset(i) === 0)
+        return window.__arrivedAtOnce === true && at(1) && items.every((_, i) => inset(i) === 0)
             && items.every((item) => Math.abs(item.getBoundingClientRect().width - size) < 1.5)
         JS));
 });