class:/style: directives can be overridden by class/style attributes

[dummdidumm]

Describe the bug

I'm not sure if this is related to #13171 or not, but class:/style: directives can be overridden by class/style attributes, which they shouldn't - the directives always take precedent. It happens if the class:/style: directive doesn't update/rerun while the corresponding class/style attribute does update.

The solution is probably to ensure that the directives are always in the same template effect as the corresponding attributes so that they rerun. Alternatively there's some hidden property on the DOM elements that tells them what other classes should be on there at that point.

Reproduction

https://svelte-5-preview.vercel.app/#H4sIAAAAAAAACl2P4WrDMAyEX0WIjaQ0kLGfmeOy51jGcB1lNXGdYMspw_jdhxfGRn_qdLr7lHAylgJ2bwmduhJ2-Lqu2CB_rWUIG1kmbDAs0euiiKC9WVkObmBLDJuykaCHh8CKqX46vAyu7KboNJvFwaj8XB9S0Qb2xNE7YB-p-DjfmT-JP7RVIdxf7DWP8AwnqG4Xw1RBB9XZKj1Xv1Gi_YNzYjQb_GR1BaFPO0jetT79q8pStKPZ5ODEOTIvDhanrdFzn-pDL_fy4zHLcDOsL6LdXRIbvC6jmQyN2JWf8nv-BkMMS5xSAQAA

Logs

No response

System Info

irrelevant

Severity

blocking an upgrade

[paoloricciuti]

Oh yeah it's my fault...I thought reverting my initial fix and using deriveds was enough but dummily didn't test for this.

But you are right, the two should be in the same template effect.

However I was confident it was not a problem because I remember a test failing for this when I made my initial fix and it was not failing with the derived.

[paoloricciuti]

Uh actually: the problem was there before because the template effects were still separate (testing this on a version before the PR still has the problem)...but I should've fixed it in my pr probably 😁

[Rich-Harris]

Working on this locally. Noticed another issue, possibly as a result of #13171 — we create an unnecessary derived when there's only a single call expression in the set of class/style directives/attributes — this...

const class_directive = $.derived(lol);

$.template_effect(() => $.toggle_class(button, "banana", $.get(class_directive)));

...should be this:

$.template_effect(() => $.toggle_class(button, "banana", lol()));

[paoloricciuti]

But when you create the class directive you can't know if while visiting the nodes we will add something else to the template_effect (for example a future set_text). But maybe we can delay the creation of the deriveds after the analysis is complete where we can check how many nodes are in the update?

[trueadm]

@Conduitry I assumed you reopened this because it's still an issue?

[Conduitry]

Yeah, it got auto closed, but what Rich said in #13353 ("The next step will be to generate code differently so as to fix #13270") marked this as closed when it presumably shouldn't have.

[Rich-Harris]

yep, see #13338