Reactivity not reflected in legacy components after Svelte 5.34.4

Why this is a good first issue

Reactivity behavior in legacy components changed in a recent version.

AI Summary

The issue involves reactivity not being reflected in legacy components after Svelte 5.34.4. The maintainer indicates this behavior is expected, but documentation or warnings might be missing. The scope involves understanding legacy component behavior and potential documentation updates.

Issue Description

Describe the bug

Deep reactivity from .svelte.js in non-runes mode components broke in 5.34.4.

I don't see anywhere in the docs that this is not supposed to work.

There are related issues to this, but they are about components that were implicitly/unexpectedly in non-runes mode:

Reproduction

https://svelte.dev/playground/78f5c3f4495b40d68def384072bce77e?version=5.50.1

Logs

System Info

System:
    OS: macOS 13.7.8
    CPU: (8) x64 Intel(R) Core(TM) i7-7700K CPU @ 4.20GHz
    Memory: 445.73 MB / 40.00 GB
    Shell: 4.3.3 - /Users/k/.nix-profile/bin/fish
  Binaries:
    Node: 24.11.1 - /Users/k/.local/share/mise/installs/node/24.11.1/bin/node
    npm: 11.6.2 - /Users/k/.local/share/mise/installs/node/24.11.1/bin/npm
    Deno: 2.5.6 - /Users/k/.local/share/mise/installs/deno/2.5.6/bin/deno
  Browsers:
    Brave Browser: 144.1.86.146
    Firefox: 147.0.3
    Safari: 18.6
  npmPackages:
    svelte: ^5.50.1 => 5.50.1

Severity

annoyance