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+++ b/testing/web-platform/tests/selection/modify-line-grid-basic.tentative.html
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+<!doctype html>
+<meta charset=utf-8>
+<title>Selection.modify(): line navigation on a grid container</title>
+<script src=/resources/testharness.js></script>
+<script src=/resources/testharnessreport.js></script>
+<link rel="author" title="Emilio Cobos Álvarez" href="mailto:emilio@crisal.io">
+<link rel="author" title="Mozilla" href="https://mozilla.org">
+<div id="container" style="display: grid; font-family: monospace">
+  <p id="one">One</p>
+  <p id="two">Two</p>
+  <p id="three">Three</p>
+  <p id="four">Four</p>
+</div>
+<script>
+const selection = getSelection();
+test(() => {
+  // Put the caret in the second character in "One"
+  selection.collapse(one.childNodes[0], 2);
+  // Move forward a line, selection should be after "Tw".
+  selection.modify("extend", "forward", "line")
+  assert_equals(selection.focusNode, two.childNodes[0]);
+  assert_equals(selection.focusOffset, 2);
+
+  // Move forward another line, selection should be after "Th".
+  selection.modify("extend", "forward", "line")
+  assert_equals(selection.focusNode, three.childNodes[0]);
+  assert_equals(selection.focusOffset, 2);
+
+  assert_equals(selection.toString().replaceAll("\r\n", "\n"), "e\n\nTwo\n\nTh");
+}, "forward");
+
+test(() => {
+  // Put the caret in the second character in "Three"
+  selection.collapse(three.childNodes[0], 2);
+  // Move backward a line, selection should be after "Tw".
+  selection.modify("extend", "backward", "line")
+  assert_equals(selection.focusNode, two.childNodes[0]);
+  assert_equals(selection.focusOffset, 2);
+
+  // Move backward another line, selection should be after "On".
+  selection.modify("extend", "backward", "line")
+  assert_equals(selection.focusNode, one.childNodes[0]);
+  assert_equals(selection.focusOffset, 2);
+
+  assert_equals(selection.toString().replaceAll("\r\n", "\n"), "e\n\nTwo\n\nTh");
+}, "backward");
+</script>