Transitions may break dependencies when they change during transition

[tfedor]

Describe the bug

I have a parent component, that changes value used in children component, and toggles its visibility based. Child component has further dependencies on its value.

When parent component hides the child component, it fetches data from server, resets child and toggles it back on. If the fetching is faster than the out transition, the dependencies in children are not updated.

It's hard to describe this, but the REPL should be easily readable.

Reproduction

https://svelte.dev/playground/83dc6b3a9d964dc9ad9c7514e90839d7?version=5.33.19

System Info

svelte 5.33.19

Severity

annoyance

[7nik]

The issue is similar to #14725 and somewhat discussed in #16019: at show = true;, {#if id} doesn't property guard and id: null slips into the child component, causing the runtime error.