Object.assign breaks reactivity

[TGlide]

Describe the bug

When using Object.assign, reactivity is no longer triggered when an item inside changes.

Reproduction

https://svelte.dev/playground/796c3698bfa240e6a5d4e9c8bf08bd1c?version=5.28.2

Logs

init count 0
init countObj 0
init countObjAssign 0
update count 1
update countObj 1
update count 2
update countObj 2
update count 3
update countObj 3

System Info

System:
    OS: Linux 6.6 cpe:/o:nixos:nixos:24.11 24.11 (Vicuna)
    CPU: (32) x64 AMD Ryzen 9 7950X3D 16-Core Processor
    Memory: 53.46 GB / 62.01 GB
    Container: Yes
    Shell: 3.7.1 - /run/current-system/sw/bin/fish
  Binaries:
    Node: 22.14.0 - /etc/profiles/per-user/thomasgl/bin/node
    Yarn: 1.22.22 - /etc/profiles/per-user/thomasgl/bin/yarn
    npm: 10.9.2 - /etc/profiles/per-user/thomasgl/bin/npm
    pnpm: 9.15.5 - /etc/profiles/per-user/thomasgl/bin/pnpm
    bun: 1.2.2 - /etc/profiles/per-user/thomasgl/bin/bun
  Browsers:
    Chromium: 133.0.6943.98
  npmPackages:
    svelte: ^5.28.2 => 5.28.2

Severity

annoyance

[Conduitry]

Object.assign() evaluates getters eagerly. This is how vanilla JS works as well. You can use Object.defineProperties if you want to add lazily evaluated getters to an existing object.