TextFormat: underlineThickness プロパティ

<div id="editor" style="height:200px;background:#eee;"></div>

const editorEl = document.getElementById("editor");
const editContext = new EditContext(editorEl);
editorEl.editContext = editContext;
editContext.addEventListener("textformatupdate", (e) => {
  const formats = e.getTextFormats();
  for (const format of formats) {
    console.log(
      `IME wants to apply a ${format.underlineThickness} underline between ${format.rangeStart} and ${format.rangeEnd}.`,
    );
  }
});