`<svelte:component>` updates props before itself

[buhrmi]

Describe the bug

When using <svelte:component>, and changing the component at the same time as its props, the old component updates with the new props, before the component is replaced with the new component.

In the provided reproduction, when clicking the "Swap Components" button, the console will show:

init prop1 prop1
update prop1 undefined
init prop2 prop2

Expected Behavior: The old component should not receive the new props, and the console output should be

init prop1 prop1
init prop2 prop2

Reproduction

https://svelte.dev/playground/hello-world?version=5.34.1#H4sIAAAAAAAAE5VSwW6DMAz9lSiaVCpVoOaIaKVpl913HDtQMFW0kETEXTch_n1JgJJ2ReoukNjPL89-7qgsGqApfQUhFDmrVlQkgoojVGu6oTUXYGj63lH80Q7nAjY-Vj1rHZsvEOhih8LAvXipJIJES0MzU7Zc4z6XOfJGqxbJi2r0ltStasgqTkp3G0tXNyh2hWIByuIEDCAl7VNkR54MFgiRZ19PAN0qbeZk5-I5uug2JTn1h5y6aO9r6pMskStJUB2PAhxZtCZjGa9JFLy4Gzq5pHMM5Xj9U2KS0bkDm15mOe0HxPgDYWCZbXuXLewjZLOfLJmHL7OZrovj2HP0JBlyhxOibVrJUvDyc9fN3ff7t3Oh50GbLBnAttA6jfCNNMX2BP1mYWNCg6935iazsDXOxtA335yz1LcQedueuDQaSowuo9h4690iBFPwU0zHufWPy2eL8tl_5bMH5LNJPvsrn43y2Y38D3sruDhzWdG0Luwi9b_timIT6gMAAA==

Logs

System Info

N/A

Severity

annoyance

[regnstr]

I encountered a similar issue in SvelteKit. When navigating between pages, the old page (the one being navigated away from) gets the data props from the new page before being dismounted/destroyed (not sure about the exact terminology) and can therefore cause type errors when a previously defined $derived() becomes undefined

Reproduction: https://stackblitz.com/edit/sveltejs-kit-bugreport-navigation?file=src%2Froutes%2Fa%2F%2Bpage.svelte

Open the console and check the output from the $inspect on page a and b when navigating between them:

init root {}
update root {foo: 'bar'}
init page a {foo: 'bar'}
update page a {baz: 'qux'}
init page b {baz: 'qux'}
update page b {foo: 'bar'}
init page a {foo: 'bar'}

Expected output:

init root {}
init page a {foo: 'bar'}
init page b {baz: 'qux'}
init page a {foo: 'bar'}

Should this be turned into a separate issue or is the root cause the same?