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+<!DOCTYPE html>
+<!-- Any copyright is dedicated to the Public Domain.
+     http://creativecommons.org/publicdomain/zero/1.0/ -->
+<html>
+  <meta charset="utf-8">
+  <title>CSS Flexbox Test: Fragmentation of block-size:auto single-line row flex container with fixed block-size flex items</title>
+  <link rel="author" title="Ting-Yu Lin" href="mailto:tlin@mozilla.com">
+  <link rel="author" title="Mozilla" href="http://www.mozilla.org/">
+  <link rel="help" href="https://drafts.csswg.org/css-flexbox-1/#pagination">
+  <link rel="match" href="flexbox-single-row-vertical-lr-1-ref.html">
+  <meta name="assert" content="This test verifies fragmentation of fixed block-size flex items in a block-size:auto single-line row flex container, with various combinations of margin/border/padding, alignments, and column-gap.  Also use a script to shrink multicol block-size.">
+
+  <style>
+  html {
+    writing-mode: vertical-lr;
+  }
+  .multicol {
+    block-size: 80px;
+    inline-size: 320px;
+    column-count: 3;
+    column-fill: auto;
+    column-gap: 10px;
+    border: 5px solid orange;
+    margin-block-end: 5px; /* Just to separate each sub-testcase visually. */
+  }
+  .flexContainer {
+    display: flex;
+    flex-wrap: nowrap;
+    flex-direction: row;
+    background: gray;
+    /* border-width is 0 by default; some sub-testcases will increase it. */
+    border: 0 solid lightgray;
+  }
+  .border-padding {
+    border-block-width: 10px;
+    padding-block: 5px;
+  }
+
+  /* Note: flex items are in visual order as "CMYK": cyan, magenta, yellow, black. */
+  header, nav, article, footer {
+    flex-grow: 1;
+  }
+  header {
+    background: cyan;
+    block-size: 100px;
+  }
+  nav {
+    background: magenta;
+    block-size: 25px;
+    align-self: flex-start;
+  }
+  article {
+    background: yellow;
+    block-size: 50px;
+    align-self: center;
+  }
+  footer {
+    background: black;
+    block-size: 75px;
+    align-self: flex-end;
+  }
+
+  body.multicol-children-block-size > .multicol {
+    block-size: 50px;
+  }
+  </style>
+
+  <script>
+  function runTest() {
+    document.body.offsetHeight;
+
+    /* Shrink the multicol's block-size to exercise pulling and pushing flex items
+       of an already fragmented flex container. */
+    document.body.classList.add("multicol-children-block-size");
+  }
+  </script>
+
+  <body onload="runTest()">
+    <!-- Basic one without any margin/border/padding. -->
+    <div class="multicol">
+      <div class="flexContainer">
+        <header></header><nav></nav><article></article><footer></footer>
+      </div>
+    </div>
+
+    <!-- Test a container with margin-block-start. -->
+    <div class="multicol">
+      <div class="flexContainer" style="margin-block-start: 25px">
+        <header></header><nav></nav><article></article><footer></footer>
+      </div>
+    </div>
+
+    <!-- Test a container not at the top of the column/page. -->
+    <div class="multicol">
+      <div style="block-size: 25px"></div>
+      <div class="flexContainer">
+        <header></header><nav></nav><article></article><footer></footer>
+      </div>
+    </div>
+
+    <!-- Test a container with border and padding. -->
+    <div class="multicol">
+      <div class="flexContainer border-padding">
+        <header></header><nav></nav><article></article><footer></footer>
+      </div>
+    </div>
+
+    <!-- Test a container with border, padding, and column-gap. -->
+    <div class="multicol">
+      <div class="flexContainer border-padding" style="column-gap: 4px">
+        <header></header><nav></nav><article></article><footer></footer>
+      </div>
+    </div>
+  </body>
+</html>