chromium/third_party/blink/web_tests/editing/pasteboard/copy-paste-pre-line-content.html

<!doctype html>
<script src="../../resources/testharness.js"></script>
<script src="../../resources/testharnessreport.js"></script>
<script src="../assert_selection.js"></script>
<script>
selection_test(
    '<textarea>foo\n|</textarea>',
    selection => {
      selection.document.execCommand('insertHTML', false, 'bar\n');
    },
    '<textarea>foo\nbar|\n</textarea>',
    'Insert a line after another line in textarea');
</script>