feat : support transition with hidden (poc)

[adiguba]

A possible solution for #9976

It's simple to show/hide a node with the hidden attribute (of style:display), but there no way to use the svelte's transition with that, and the {#if} block destroys its contents every time the condition changes, which is not necessarily desirable.

This PR proposes a new modifier for the transition: directive : hidden.
With this modifier, the transition will only be applied when the hidden attribute is modified.

Example :

<div transition:slide|hidden={options} hidden={condition}>

</div>

This would allow to show/hide elements using Svelte transitions.

Demo REPL here (edit : modified REPL to use |this instead of |hidden)

Remarks :

• I don't really like the modifier's name hidden, but I couldn't find a better name...
• The implementation is quite simple.
• I just realized that this doesn't work with spreading, but I think it should be relatively easy to implement.
• Compatible SSR without change, as the hidden attribute will be set.
• Caveat : the hidden attribute is overridden by any CSS display value.
  It's required to have a global CSS like that :

[hidden] {
    display: none !important;
}

Is it worth it?

Svelte 5 rewrite

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• Ideally, include a test that fails without this PR but passes with it.

Tests and linting

• Run the tests with pnpm test and lint the project with pnpm lint

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[adiguba]

For the naming, perhaps we could use different directive, for example by replacing transition:/in:/out: with someting like view:/show:/hide:

-<div transition:slide|hidden={options} hidden={condition}></div>
+<div view:slide={options} hidden={condition}></div>

-<div in:slide|hidden={options} out:fade|hidden={options} hidden={condition}></div>
+<div show:slide={options} hide:fade={options} hidden={condition}></div>

[adiguba]

I fixed some bug with in: & out: directive, and support for spreading with set_attributes().
I also changed the modifier from |hidden to |this which I think is a little more explicit...

[adiguba]

Just see #13612
I didn't know that hidden could take other values, which conflicts a bit with this PR... :/

[adiguba]

closed as issue is closed and hidden may not be the best choice for that