Make outroing elements `inert`

[Rich-Harris]

Describe the problem

An element that is being removed from the DOM is still interactive, and still part of the accessibility tree. This means that (for example) it's possible to accidentally invoke a click handler on an element that should be 'dead'.

Since transitions are purely visual flourishes, it doesn't really make sense that outroing elements are still accessible.

Describe the proposed solution

The good news is that there's an inert property that can be applied to elements, which has exactly the desired effect.

The bad news is that it isn't supported everywhere.

The good news is that it will be supported everywhere when Firefox 112 comes out next week.

Since this is a breaking change (theoretically someone could be relying on the chaotic behaviour?), it ought to be part of Svelte 4 anyway, so the timing will work out nicely.

Alternatives considered

Not doing this

Importance

nice to have

[benmccann]

Implemented

[Laurens256]

it would be nice to mention this in the transition docs. I was applying my own styles based on whether an element was inert or not and was wondering why those styles were applied when transitioning out. Easy to figure out but a small mention in the docs would have been nice