fix: Fix subtitles not added to DOM region

[david-hm-morgan]

Proposed fix closes #4680

Reinstate previously removed region elements when next caption finds it is not there: Detect the absence and ensure updateDOM is true so its reinstated and the next caption can be shown.

[github-actions]

Incremental code coverage: 85.71%

[joeyparrish]

I think this is a good fix overall. Thanks! If I have permission to edit, I'll make the change I suggested.

[joeyparrish]

With only one parent element, I think this could be rewritten as a while loop and be slightly more efficient that way.

https://jsbench.me/ suggests it would be 10% faster, though admittedly I did put the target element 100 levels deep in my test case.

Setup:

  const myContainer = document.createElement('div');
  let innerHTML = '<span id="myTarget"></span>';
  for (i = 0; i < 100; i++) {
    innerHTML = `<span>${innerHTML}</span>`;
  }
  myContainer.innerHTML = innerHTML;
  document.body.appendChild(myContainer);

Recursive:

function v1(thing, ancestor) {
  if (thing == null) {
    return false;
  }
  if (thing == ancestor) {
    return true;
  }
  return v1(thing.parentElement, ancestor);
}

console.log(v1(myTarget, document.body));

Loop:

function v2(thing, ancestor) {
  while (thing != null) {
    if (thing == ancestor) {
      return true;
    }
    thing = thing.parentElement;
  }
  return false;
}

console.log(v2(myTarget, document.body));