fix: wait a microtask for await blocks to reduce UI churn

[trueadm]

The PR changes await blocks, to avoid showing/hiding the pending/then state of an each block if the promise resolves by the next microtask. Fixes #8459.

[changeset-bot]

🦋 Changeset detected

Latest commit: 99dcc68

The changes in this PR will be included in the next version bump.

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svelte	Patch

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[dummdidumm]

Are you sure this works? I still get the pending block logged in this example

[trueadm]

Are you sure this works? I still get the pending block logged in this example

This PR aims to reduce the amount of churn on pending blocks. The then block is an entirely different beast that will involve majorly refactoring how the then block works to use signals. Right now it works a bit like the key block.

[dummdidumm]

The referenced issue states:

When the promise of an {#await ... } block changes from a resolved Promise to another resolved Promise, svelte unnecessarily unmounts and then remounts all the contents of the "then" fragment of the await block.

... which is what is still happening here. Switching from one resolved promise to another is the main thing that people complain about.

[trueadm]

The referenced issue states:

When the promise of an {#await ... } block changes from a resolved Promise to another resolved Promise, svelte unnecessarily unmounts and then remounts all the contents of the "then" fragment of the await block.

... which is what is still happening here. Switching from one resolved promise to another is the main thing that people complain about.

I've updated the PR to reflect that this isn't solving that issue anymore.

[dummdidumm]

But what is this PR really changing then? The pending block is still rendered on promise change, just the effect isn't fired because it's cleaned up before it can run.

[trueadm]

@dummdidumm The pending block isn't rendered on promise change if it only takes a microtask (except initial mount). There was a bug in your case, but I just fixed that.

[dummdidumm]

With your change the referenced issue is fixed, and my playground works as expected. I assume this was a "order in the task queue" issue and adding the .then before queuing the microtask fixed it?

[Rich-Harris]

Why render the pending block on mount, if the promise is already resolved? Opened #11995 as a proof of concept of not doing that

[trueadm]

Converted this to a draft. I think I can get then blocks to also not remount too.

Update: ready now!

[Rich-Harris]

The pending block is destroyed and recreated if you go from one unresolved promise to another. Working on it locally