Modal improvements

[Atmos4]

Scrolling improvements

• overflow on modal instead of article
• prevent body overflow when modal is open

Animations

• animations on open and close. Uses allow-discrete to prevent the modal from being focusable when closed. Browser support is low but non-support will just result in graceful degradation (no animation on closing modal).

This PR is more of a POC. I can split it into 2 PRs, one for the scrolling and one for the animations.

I am also unsure which animations I should include. Maybe it is just safer to explain in the docs how to add animations easily rather than enforcing specific animations.

[lucaslarroche]

@Atmos4, thanks a lot! And sorry for the delay.

I like the change to move overflow to the modal instead of the inner article. We can keep everything in one PR.

I wasn’t familiar with allow-discrete — does it allow entry and exit animations without JS? (Ref: modal.js)

For animations, we should stick to the existing ones defined in the utility classes: .modal-is-opening and .modal-is-closing.