fix: resolve legacy component props equality for mutations

[trueadm]

Fixes #12235

[changeset-bot]

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Latest commit: 82983fd

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

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[Rich-Harris]

hold up, i don't think this is right

[Rich-Harris]

Added the 'request changes' because I was still investigating and wanted to make sure we didn't merge hastily.

My concern was that this might be addressing the symptom rather than the cause, which is that sources inside a mutable proxy can diverge from their ground truth — in fact it's fixable with a much smaller diff:

get(target, prop, receiver) {
  // ...

  if (s !== undefined) {
+   if (!metadata.i && metadata.p !== s.v) {
+     // in mutable mode, it's possible for the source to diverge from the ground truth
+     s.v = proxy(metadata.t[prop]);
    }

    const value = get(s);
    return value === UNINITIALIZED ? undefined : value;
  }

  return Reflect.get(target, prop, receiver);
}

But I was wrong, because this is the only place where proxies can be mutable. I'm still going to make a couple of tweaks though:

• we can make the test slightly simpler (one of the problems with our test suite is that tests often involve lots of moving parts, making it trickier to pinpoint the thing they're actually testing. In this case, we don't need to use an each block. This is top of mind for me at the moment because of feat: single-pass hydration #12335, which featured a lot of red herring test failures until everything worked perfectly)
• we can now get rid of mutable proxies!