fix: apply overflow: hidden style when transitioning elements, where necessary

[Rich-Harris]

This is a slightly hacky fix for #4712. It works by adding style="overflow: hidden" on transitioning elements, if the transition includes overflow: hidden (which is the case for slide, for example).

We could argue that we shouldn't bother with this, since it's fixed in Safari 18. I just wanted to see how easy the fix would be.

Before submitting the PR, please make sure you do the following

• It's really useful if your PR references an issue where it is discussed ahead of time. In many cases, features are absent for a reason. For large changes, please create an RFC: https://github.com/sveltejs/rfcs
• Prefix your PR title with feat:, fix:, chore:, or docs:.
• This message body should clearly illustrate what problems it solves.
• Ideally, include a test that fails without this PR but passes with it.
• If this PR changes code within packages/svelte/src, add a changeset (npx changeset).

Tests and linting

• Run the tests with pnpm test and lint the project with pnpm lint

[changeset-bot]

🦋 Changeset detected

Latest commit: 2cd81c7

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
svelte	Patch

Not sure what this means? Click here to learn what changesets are.

Click here if you're a maintainer who wants to add another changeset to this PR

[github-actions]

Playground

pnpm add https://pkg.pr.new/svelte@14930

[Rich-Harris]

preview: https://svelte-dev-git-preview-svelte-14930-svelte.vercel.app/

this is an automated message

[trueadm]

I think this is still a worthwhile fix for older versions of Safari. However, I'll delegate to you if you still want to merge this.

[Rich-Harris]

it's only a small addition and it closes an issue, i'll merge it