Address mouse events over disabled form fields

[Boldewyn]

This might be an interesting case to address for Hyperform: Try to fix the behaviour mentioned by @jakearchibald, tweet and blog post:

Turns out, mouse events don't fire when the pointer is over disabled form elements, except in Firefox.

and his proposed fix:

input[disabled],
button[disabled] {
  pointer-events: none;
}

This means disabled form elements cannot be interacted with, but it also solves the event issue.

We could implement that in a similar fashion to how we handled the button > span case in d855e6d.