fix(compat): watch arrays as deep: 1

[skirtles-code]

• Fix #11872.
• Close #12236.

This PR builds on the work done in #12236 to try to make the compat build more accurately emulate Vue 2.

Watching arrays in Vue 2 behaved like deep: 1, rather than deep: true, so mutating nested objects should not trigger the watcher. Support for a numeric depth was added in 3.5 and wasn't available when the compat build was first implemented.

But there are several other inconsistencies in the compat build. Some of these seem to have been introduced by #9927, and worked correctly prior to 3.5:

• The special handling for arrays should not depend on the initial value of the property, only the current value needs to be an array. In practice, it's quite common for a property to have an initial value of null while data is loading.
• The special handling is also supposed to apply to this.$watch(), not just the watch option.
• The compat warning should not be shown when deep: true has been set.

There were relatively few tests for WATCH_ARRAY, so it's understandable that these regressions were introduced. I've added a lot of extra tests to try to avoid future problems.

I've introduced a compatWatchArray flag to the internal watcher options as a way to tell @vue/reactivity that we want the special handling of arrays. This only impacts the equality check, which needs to behave like a deep watcher only if the current value is an array. I didn't want to add this flag to the public types, so there's a bit of as any hackery to access it.

Summary by CodeRabbit

• New Features

  • Improved compatibility for watching arrays, ensuring watchers are triggered as expected when using legacy array watching behavior in compatibility mode.

• Bug Fixes

  • Enhanced handling of array watchers to better support legacy scenarios and prevent missed updates.

• Tests

  • Expanded and reorganized test coverage for array watching compatibility, including more scenarios and dynamic behaviors.

[coderabbitai[bot]]

Walkthrough

Compatibility logic for array watching was added and refactored across the reactivity and runtime-core modules. The watch callback triggering conditions were extended to support legacy array semantics under a compatibility flag. The watcher getter logic was centralized and reused, and the test suite for the WATCH_ARRAY deprecation was significantly expanded to cover more scenarios.

Changes

File(s)	Change Summary
packages/reactivity/src/watch.ts	Extended watch callback triggering to include compat mode with compatWatchArray for arrays.
packages/runtime-core/src/apiWatch.ts	Added createCompatWatchGetter for compat array watching; updated instanceWatch for compat array logic.
packages/runtime-core/src/componentOptions.ts	Refactored compat array watch logic to use createCompatWatchGetter; made createWatcher internal.
packages/vue-compat/tests/misc.spec.ts	Replaced single test with comprehensive suite for WATCH_ARRAY deprecation and compat behaviors.

Sequence Diagram(s)

sequenceDiagram
    participant Component
    participant Watcher
    participant CompatGetter
    participant Callback

    Component->>Watcher: Register watch on array property (compat mode)
    Watcher->>CompatGetter: Get value (array)
    CompatGetter->>Watcher: Return deeply traversed array (depth 1)
    Watcher->>Callback: Invoke callback if compatWatchArray flag and array detected

Poem

A bunny with code in its paws,
Watches arrays with legacy laws.
Compat flags set, old flows revive,
Deep or shallow, the tests now thrive.
With every tick, the watcher’s aware—
Arrays in harmony, handled with care!
🐇✨

✨ Finishing Touches

• [ ] 📝 Generate Docstrings

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commit: 351ce06

[github-actions[bot]]

Size Report

Bundles

File	Size	Gzip	Brotli
runtime-dom.global.prod.js	101 kB	38.3 kB	34.5 kB
vue.global.prod.js	159 kB	58.5 kB	52 kB

Usages

Name	Size	Gzip	Brotli
createApp (CAPI only)	46.6 kB	18.2 kB	16.7 kB
createApp	54.5 kB	21.2 kB	19.4 kB
createSSRApp	58.8 kB	23 kB	20.9 kB
defineCustomElement	59.5 kB	22.8 kB	20.8 kB
overall	68.6 kB	26.4 kB	24.1 kB

[edison1105]

/ecosystem-ci run

[vue-bot]

📝 Ran ecosystem CI: Open

suite	result	latest scheduled
pinia	:white_check_mark: success	:white_check_mark: success
nuxt	:white_check_mark: success	:white_check_mark: success
language-tools	:white_check_mark: success	:white_check_mark: success
radix-vue	:white_check_mark: success	:white_check_mark: success
primevue	:white_check_mark: success	:white_check_mark: success
vitepress	:white_check_mark: success	:white_check_mark: success
quasar	:white_check_mark: success	:white_check_mark: success
vueuse	:white_check_mark: success	:white_check_mark: success
vue-i18n	:white_check_mark: success	:white_check_mark: success
vite-plugin-vue	:white_check_mark: success	:white_check_mark: success
vant	:white_check_mark: success	:white_check_mark: success
router	:white_check_mark: success	:white_check_mark: success
test-utils	:white_check_mark: success	:white_check_mark: success
vue-simple-compiler	:white_check_mark: success	:white_check_mark: success
vuetify	:white_check_mark: success	:white_check_mark: success
vue-macros	:white_check_mark: success	:white_check_mark: success

[edison1105]

@skirtles-code the tests are all green after re-run. I think the document https://v3-migration.vuejs.org/breaking-changes/watch.html should also be updated. It is also possible that it should be merged into the minor branch This PR looks good to me.