chore: perf tweaks for actions/styles/classes

[dummdidumm]

• check if we really need to add/remove the class (calling includes first is cheaper than always setting/removing it)
• check if we really need to update a style (calling getPropertyValue/setProperty is expensive)
• check if we should call the action's update function (this is not only a perf tweak but also a correctness fix)

closes #12652

@peterkogo could you check if these tweaks increase perf for you? From a quick local test it did for me

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🦋 Changeset detected

Latest commit: 0348817

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[peterkogo]

Wow, that was quick! I'll check first thing tomorrow morning!

[peterkogo]

@dummdidumm okay I couldn't wait.
It already looks much better on my end!
However, my action update functions are still re-running. Double-checked to see if I was tripping up on something, and it appears that all of my derived, effects etc. are rerunning on an update — which is the reason all of my actions get updated constantly.

Am I mistaken in the assumption that this should not happen? REPL with $inspect(id) rerunning on every update.

[dummdidumm]

How does your action update function look like? Does it take an object or a primitive? Can you give an example REPL using an action where things update which you don't expect to / which don't update in Svelte 4?

In general, the update behavior is a bit different in Svelte 5. Since you're using $state.frozen, you're updating the object as a whole through one signal, and so everything depending on that signal updates, even if the value within the signal it listens to hasn't changed. To avoid that, you can add $derived in the right places.

[peterkogo]

I am calling the action with an object containing a mixture of props and some other stuff.
I was not aware that immutable state behaves like that in Svelte 5 and thought that the reason <svelte:options immutable /> was kicked because this is of no concern anymore.
You think it makes sense for me to open a separate issue on this topic? I'm not too keen on having to memoize every prop by hand 🥲