Reactivity with CSS Variables does not work when spreading restProps

[canastro]

Describe the bug

Given a component that creates a inline CSS variable from a given prop (and spreads the $$restProps)
When the prop changes
Expected the CSS variable is updated
Actual the CSS variable does not update

The key here is "and spreads the $$restProps", if you remove this line, the issue does not occur anymore (see REPL)

I can't really be sure if this is the only particular case where this happens, but this is the scenario in which I'm able to replicate.

Reproduction

I created this simple REPL to reproduce the issue:
https://svelte.dev/repl/906ddb7ef8f3436095ca0e401e0fe8b9?version=3.58.0

The toggle button should change the backgroundColor provided to the Box component. The Box component should create a CSS variable --bg-color and the background of the div should change.

In the current state of the REPL this does not work, but if you delete {...$$restProps} in the Box.svelte everything works as expected.

Logs

No response

System Info

System:
    OS: macOS 13.2.1
    CPU: (10) arm64 Apple M1 Pro
    Memory: 138.67 MB / 32.00 GB
    Shell: 5.8.1 - /bin/zsh
  Binaries:
    Node: 16.17.1 - ~/Library/pnpm/node
    npm: 8.15.0 - ~/Library/pnpm/npm
  Browsers:
    Chrome: 111.0.5563.146
    Edge: 111.0.1661.54
    Firefox: 108.0.1
    Safari: 16.3

Severity

annoyance

Not sure what to put here, in our case is blocking the way we implemented some features.

[KutayGuler]

Hi, I'm not sure what your end goal is but here is a possible fix:

https://svelte.dev/repl/46c7e6dd457d44f8a1a9086ba88ebc22?version=3.58.0

[canastro]

Yeah, but we actually want these to be passed as component props to be sure they are typed accordingly to our token types and to have a clear and concise API of what our component supports as customization.

[Conduitry]

This has been fixed in 3.59.0 - https://svelte.dev/repl/906ddb7ef8f3436095ca0e401e0fe8b9?version=3.59.0