Classes set imperatively (by actions) are removed when class directive updates

[Jibbedi]

Describe the bug

I have an action that sets a class.

When i use a class directive that contain reactive values, the class set by the action is removed when the value updates.

I'm not sure if this is actually a bug or the expected behavior, since one could argue that the source of truth for the classes should be handled only by the component.

However i feel like actions kind of encourage classes to be added by them.

The problem only occurs for dynamic values inside class

class={"Button" + (small ? "small": "large")}

using a class:small directive for example does not remove the action class.

Is it possible to parse out any static value and continue to set them via $.set_class
while dynamic values get treated differently? At least for static analyzable values like in the example above the compiler could probably generate something like this?

const small = $.get(small);
$.toggle_class(button_1, "small", small);
$.toggle_class(button_1, "large", !small);

Reproduction

https://svelte-5-preview.vercel.app/#H4sIAAAAAAAAE5WOQUvEMBCF_8o4eGix7Hqu7cqqR4-CB-MhptMlbJqUzlSQkv8uSdC9CZ4m8-blfW_D0TpibN829HoibPE4z9igfM1p4U9yQtggh3UxSenYLHaWg_JKHAmYsHqBHq5ZtFB1W98pn27j6o3Y4EHnUfkwUA1bOilJy844zfxsWXZ6GCqFx2x8TKrCOhljiUoYnrRzF8yoHVNCdftLH999rCLBw8rUFixkSJt_91sesWj9pvAh2xVGCN44a879VtXQH0rNH-RVfigf4-ElnE6O4DUsZ-72hfYn-UIBhXADVQm9B4UlFVtQ6PRyIoX1f3s8BWLwQXKh3z7Y4BQGO1oasJVlpfgevwGNOoQQ6AEAAA==

Logs

No response

System Info

System:
    OS: macOS 14.4.1
    CPU: (10) arm64 Apple M1 Max
    Memory: 33.33 GB / 64.00 GB
    Shell: 5.9 - /bin/zsh
  Binaries:
    Node: 20.11.1 - ~/.volta/tools/image/node/20.11.1/bin/node
    Yarn: 1.22.19 - ~/.volta/tools/image/yarn/1.22.19/bin/yarn
    npm: 10.2.4 - ~/.volta/tools/image/node/20.11.1/bin/npm
    bun: 0.4.0 - ~/.bun/bin/bun
  Browsers:
    Brave Browser: 119.1.60.118
    Chrome: 129.0.6668.60
    Chrome Canary: 131.0.6740.0
    Edge: 129.0.2792.52
    Safari: 17.4.1
  npmPackages:
    svelte: ^5.0.0-next.259

Severity

annoyance

[paoloricciuti]

Duplicate of #13270

Thanks for reporting tho 😄