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+<!doctype html>
+<meta charset="utf-8">
+<title>Tests that after making a scrolling box a containing block, dynamically-inserted abspos boxes still get positioned correctly</title>
+<link rel="help" href="https://bugzilla.mozilla.org/show_bug.cgi?id=1735533">
+<link rel="help" href="https://drafts.csswg.org/css-position/#abspos-insets">
+<link rel="author" href="mailto:emilio@crisal.io" title="Emilio Cobos Álvarez">
+<link rel="author" href="https://mozilla.org" title="Mozilla">
+<link rel="match" href="containing-block-change-scrollframe-ref.html">
+<style>
+  #container {
+    width: 400px;
+    height: 400px;
+    margin: 100px;
+    background-color: purple;
+    overflow: hidden;
+  }
+  #spacer {
+    height: 700px;
+  }
+  #bottom {
+    height: 100px;
+    background-color: green;
+  }
+  #abspos {
+    position: absolute;
+    width: 100px;
+    height: 100px;
+    background-color: green;
+    top: 400px;
+    left: 0;
+  }
+</style>
+<div id="container">
+  <div id="spacer"></div>
+  <div id="bottom"></div>
+</div>
+<script>
+  let container = document.getElementById("container");
+  container.scrollTop = 400;
+  container.getBoundingClientRect();
+  container.style.position = "relative";
+  container.getBoundingClientRect();
+  let abspos = document.createElement("div");
+  abspos.id = "abspos";
+  container.appendChild(abspos);
+</script>