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diff --git a/layout/reftests/transform/partial-prerender-translate-4.html b/layout/reftests/transform/partial-prerender-translate-4.html
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+<!DOCTYPE html>
+<html class="reftest-wait reftest-no-flush">
+<meta name="viewport" content="width=device-width,minimum-scale=1,initial-scale=1">
+<!--
+  A test that a partial pre-rendered transform animation is transformed by an
+  ancestor transform value.
+  If we don't factor the ancestor's transform, we mis-recognize janks, which
+  means janked at the initial translateX(0vw) position, thus white blank area
+  will appears.
+  -->
+<style>
+html {
+  overflow: hidden;
+}
+body {
+  margin: 0px;
+  padding: 0px;
+}
+@keyframes anim {
+  /* translate to a position where the left edge of partial pre-rendered on */
+  /* the left edge of the viewport. */
+  to { transform: translateX(-50vw); }
+}
+#target {
+  width: 300vw;
+  height: 300vh;
+  transform: translateX(0vw);
+}
+</style>
+<div style="transform: translateX(50vw);">
+  <div id="target">
+    <!--
+      Put an SVG element so that the transform display item has a blob on
+      WebRender which means this test properly fails without the proper fix.
+      -->
+    <svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 16 16">
+      <rect fill="green" width="16" height="16"></rect>
+    </svg>
+  </div>
+</div>
+<script>
+document.addEventListener("MozReftestInvalidate", () => {
+  target.style.animation = "anim 100s 1s step-start";
+  target.addEventListener("animationstart", () => {
+    // animtionstart event is fired just before requestAnimationFrame callbacks,
+    // so we need to wait two rAF to make sure the initial animation value is
+    // composited on the compositor.
+    requestAnimationFrame(() => {
+      requestAnimationFrame(() => {
+        document.documentElement.classList.remove("reftest-wait");
+      });
+    });
+  });
+}, { once: true });
+</script>
+</html>