https://github.com/sveltejs/svelte/issues/1037#issuecomment-737872461
Explanation (from https://github.com/sveltejs/svelte/issues/1037#issuecomment-739458005):
@AlexGalays register is an action created and passed in from the parent node (Wrapper) which allows the child to register with it. Not builtin to svelte.
That's very clever @PatrickG. Nice one. I was a bit confused when first looking at it to understand what was going on, but I think that will be a handy tool in the toolbox.
But why do we need this? If we remove all use:register, it still toggles just fine. Seems the only benefit is that this allows cleanup.