fix: ensure custom elements do not sync flush on mount

[trueadm]

Fixes #12772.

We currently have an inconsistency around using $effect together with custom elements. If you put in an $effect inside a custom element, then we flush the effects sync before moving on the handling the next custom element. This is problematic when you have things like event handlers or other sequence/propagation sensitive things around as normally effects happen after we write we've handled the DOM, not incrementally as we're handling the DOM.

This is compounded if someone were to have $effect.pre in one custom element that is a parent of a child that has an $effect, the child $effect would run before the parent $effect.pre which makes for glitchy behaviour when creating interactions that depend on the correct ordering.

To fix this, we remove the problematic flush_sync from Svelte4Component. If people want to force their custom elements to work sync, they can always manually flush_sync themselves, however that's rarely needed.

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[dummdidumm]

This would be a big breaking change for everyone else, I don't think we can do that. If we determine this needs to be fixed for custom elements somehow, then the flush_sync should only be avoided for the custom element wrappers

[trueadm]

This would be a big breaking change for everyone else, I don't think we can do that. If we determine this needs to be fixed for custom elements somehow, then the flush_sync should only be avoided for the custom element wrappers

Updated it so it only happens for non custom-element wrappers.

[trueadm]

I tweaked the test so it fails on main. The event handler needs to be added on after the first tick to emulate another custom element adding the event handler.

[Rich-Harris]

ah whoops, gotcha — thought it was just about delaying the check until both the element upgrade and the effect