s.js

const { focusBlock } = editor.value
if (
  focusBlock.type === BlockTypes.h1 ||
  focusBlock.type === BlockTypes.h2
) {
  const selection = editor.value.selection
  if (
    selection.isCollapsed &&
    selection.focus.offset === focusBlock.text.length
  ) {
    editor.splitBlock(1).setBlocks(BlockTypes.paragraph)
    return
  } else {
    return editor.insertText('\n')
  }
}