1 files changed, 52 insertions, 0 deletions
diff --git a/testing/web-platform/tests/scroll-animations/css/scroll-timeline-sampling.html b/testing/web-platform/tests/scroll-animations/css/scroll-timeline-sampling.html
new file mode 100644
index 0000000000..a67f3b94e4
--- /dev/null
+++ b/testing/web-platform/tests/scroll-animations/css/scroll-timeline-sampling.html
@@ -0,0 +1,52 @@
+<!DOCTYPE html>
+<link rel="help" src="https://drafts.csswg.org/scroll-animations-1/#avoiding-cycles">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="/web-animations/testcommon.js"></script>
+<style>
+  #scroller {
+    overflow: hidden;
+    width: 100px;
+    height: 100px;
+    scroll-timeline: --timeline;
+  }
+  #contents {
+    height: 200px;
+  }
+  @keyframes expand {
+    from { width: 100px; }
+    to { width: 200px; }
+  }
+  #element {
+    width: 0px;
+  }
+  #element.animate {
+    animation: expand 10s linear;
+    animation-timeline: --timeline;
+  }
+  /* Ensure stable expectations if feature is not supported */
+  @supports not (animation-timeline:--foo) {
+    #element { animation-play-state: paused; }
+  }
+</style>
+<div id=scroller>
+  <div id=contents></div>
+  <div id=element></div>
+</div>
+<script>
+  promise_test(async (t) => {
+    assert_equals(getComputedStyle(element).width, '0px');
+    await runAndWaitForFrameUpdate(() => {
+      element.classList.add('animate');
+    });
+    assert_equals(getComputedStyle(element).width, '100px');
+
+    scroller.scrollTop = 50;
+    // Scrolling position should not yet be reflected in the animation,
+    // since the new scroll position has not yet been sampled.
+    assert_equals(getComputedStyle(element).width, '100px');
+
+    await waitForNextFrame();
+    assert_equals(getComputedStyle(element).width, '150px');
+  }, 'Scroll position is sampled once per frame');
+</script>