Highlight Matrix block that is currently being edited.

[thupsi]

As a follow-up to #18484 (looks a lot cleaner, thanks!):

Can we have the Matrix block that is currently being edited highlighted in some way?

Example CSS:

.matrixblock {
  &:focus-within {
    outline: 2px solid var(--link-color);
  }
}

Right now, same color matrix blocks kinda blend with each other, making it a little difficult for authors to identify the beggining and end of each block:

Even the small outline from the CSS above makes a meaningful difference:



As a bonus (and I'm not sure of the best way to do this, or if there are a11y implications) even clicking anywhere inside the block could trigger the outline.

That's it, thanks!