fix(reactivity): ensure watch(Effect) can run independent of unmounted instance if created in a detatched effectScope (fix #7319)

[LinusBorg]

close #7319

[LinusBorg]

/cc @posva this should solve your problem in pinia.

[posva]

Nice! I think it would be even better to have 2 different count variables as it would make the test more robust

[LinusBorg]

Good call, will do.

[yangmingshan]

I think this PR may have hidden issues, especially for undetached effectScope. There are many other functions depends on currentInstance, like callWithErrorHandling queuePostRenderEffect etc.

watchEffect should be same as watch, maybe we can remove instance unmounted short circuit in watchEffect, since watch didn't have the same logic? I tried, no unit test fail.

If we need keep the short circuit for some reason, I think the better fix is:

getter = () => {
-  if (instance && instance.isUnmounted) {
+  if (currentScope && !currentScope.active) {
     return
  }
}

I also tried, all unit tests passed.

cc @yyx990803

[yyx990803]

@yangmingshan PR welcome so we can take a better look and run ecosystem-ci if needed.