LayoutTests/fast/editing/partial-layout-when-inserting-content.html

<style>
div {
  font-size: 20px;
  font-family: Monospace;
  width: 150px;
  outline: none;
}
</style>
<div id=edit_this contenteditable="true">first secondline</div>
<script>
document.body.offsetHeight;
let range = new Range();
range.setStart(edit_this.firstChild, 12);

let selection = window.getSelection();
selection.removeAllRanges();
selection.addRange(range);

edit_this.focus();
document.execCommand("insertText", false, " ");
edit_this.blur();
</script>