<div>
  <textarea readonly style="display: block; width: calc(100vw - 16px); max-width: calc(100vw - 16px); height: calc(100vh - 16px); max-height: calc(100vh - 16px); outline: none;"></textarea>
</div>

*,
*::before,
*::after {
 box-sizing: border-box;
}


div {
  position: relative;
  width: fit-content;

  &::before {
    content: "";
    display: block;
    position: absolute;
    width: 100%;
    height: 100%;
    background-image: linear-gradient(to top right, transparent calc(50% - 1px), black, transparent calc(50% + 1px));
    pointer-events: none;
  }
}