feat: allow non-synchronous legacy component instantiation

[dummdidumm]

Add a new option to the legacy class component interface so that flush_sync can be omitted. Part of sveltejs/kit#12248

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🦋 Changeset detected

Latest commit: 461b506

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

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[trueadm]

Couldn't we just tell people to use flushSync after? Does that work?

[dummdidumm]

No, it would be a needless potentially hard to detect breaking change. The default for legacy should stay sync

[l1bbcsg]

Is this actually implemented correctly?

Comment says flushSync should not run if user opted out, but then the code actually runs it precisely when it was set to false. And setting it to true passes the control to $$host check. Furthermore, default value is undefined which is not === false.

It's simply not possible to control flushSync with sync option, this seems like a typo.
Shouldn't the predicate be like this? Note the extra parenthesis.

if (!(options?.props?.$$host || options.sync === false)) { 
  flushSync();
}

Or, easier to read:

if (!options?.props?.$$host && options.sync !== false) { 
  flushSync();
}