fix(reactivity): fix WATCH_ARRAY v2 compat deep watching elements (fix #11872)

[thecodewarrior]

close #11872

This makes a minor change that brings the WATCH_ARRAY compat behavior further in line with the Vue 2 behavior.

Currently, watching an array with WATCH_ARRAY compat enabled will trigger the watch function on deep mutation of elements. This differs from the Vue 2 behavior, where the watch would trigger when array elements were added/removed/replaced, but not mutated.

Changes:

1. Changes the WATCH_ARRAY compat to do a depth=1 traverse, meaning it will no longer create reactive dependencies on the contents of the elements.
2. Removes the getter call and isArray+isCompatEnabled check that was done before wrapping the getter, for a couple reasons:
   • The wrapped getter will be called immediately by watch anyway, so this pre-check is redundant.
   • The previous behavior of setting options.deep = true based on the initial value would lead to incorrect behavior if the initial value was an array but a future value wasn't.

I couldn't find a way to test this change directly, however in an internal project an identical change using patch-package is working beautifully.

Also as a proof of concept that depth=1 would work I forked the reproduction example from #11872, disabled the WATCH_ARRAY compat, and set the watch to use deep: 1: https://stackblitz.com/edit/vitejs-vite-qarbhv?file=src%2FApp.vue

[github-actions]

Size Report

Bundles

File	Size	Gzip	Brotli
runtime-dom.global.prod.js	100 kB	38 kB	34.2 kB
vue.global.prod.js	159 kB	57.9 kB	51.4 kB

Usages

Name	Size	Gzip	Brotli
createApp (CAPI only)	46.9 kB	18.3 kB	16.7 kB
createApp	55 kB	21.3 kB	19.4 kB
createSSRApp	59 kB	23 kB	20.9 kB
defineCustomElement	59.8 kB	22.8 kB	20.8 kB
overall	68.7 kB	26.3 kB	24 kB

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commit: 9714124

[thecodewarrior]

Re-reading the watch.ts patch, I realized that the unwrap step wouldn't unwrap arrays when isMultiSource is true. That won't be an issue however since the compat code path never creates a multi-source watch (the source parameter is only ever the single getter, never an array of sources).

[thecodewarrior]

Any updates on this? Currently we're having to resort to using patch-package to fix this, and I don't want this PR to wind up falling through the cracks.