fix: render reaction would dispose too early if observable data was changed before first useEffect

[mweststrate]

Fixed an issue where, if an observable changed between initial render and useEffect, the render reaction would be disposed. This caused any computed values to be cleaned up as well, and become untracked.

It was a bit tricky to reproduce in a unit tests, as they don't run effects async, but see the following screenshots of before and after, and the unit test.

In the screenshot, observerHeader uses amount of todo's left.
Then directly after mount, but before the useEffect happened (create HEADER), we would first change the collection (causing the render reaction to be disposed), and then set up an unrelated reaction. Since the computed value is suspended at that point, it will recomputed.

After the fix, todo's is computed only once, as should be done.

It seem that in the code somebody already thought about this case before, as a field changedBeforeMount was introduced earlier, but it was never set or read.

I've made those fields non optional; a) it would probably have prevented this bug, and b) this is in general faster as as JS engines optimize objects with a fixed shape better.

[changeset-bot]

🦋 Changeset detected

Latest commit: cabaa0a

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

This PR includes changesets to release 1 package

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mobx-react-lite	Patch

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[coveralls]

Coverage increased (+0.03%) to 93.717% when pulling cabaa0a on fix-observable-change-before-mount into c6c78b3 on master.