fix(hydration): handle transition appear hydration edge case

[edison1105]

close #13335

Summary by CodeRabbit

• New Features
  • Improved class attribute handling during hydration, resulting in more accurate detection of class mismatches on elements.
• Bug Fixes
  • Enhanced reliability when checking for class mismatches, reducing potential hydration inconsistencies.
• Tests
  • Added a test to ensure class attributes are correctly preserved and combined during transition hydration with pre-existing classes.

[coderabbitai]

"""

Walkthrough

The changes implement a caching mechanism for the class attribute during hydration, specifically storing the initial class value on the first child of a hydrated <template>. The propHasMismatch function is updated to utilize this cached value for more accurate class mismatch detection, with the element type signature adjusted accordingly.

Changes

File(s)	Change Summary
packages/runtime-core/src/hydration.ts	Added caching of initial class attribute as $cls on first child of hydrated <template> elements; updated propHasMismatch to use this cache and revised its type signature.
packages/runtime-core/tests/hydration.spec.ts	Added test case verifying that <Transition appear> preserves pre-existing classes during hydration without mismatch warnings.

Sequence Diagram(s)

sequenceDiagram
    participant Hydrator as Hydration Process
    participant Template as <template> Element
    participant Child as First Child Element
    participant MismatchCheck as propHasMismatch

    Hydrator->>Template: Hydrate content
    Template->>Child: Assign $cls with initial class attribute
    Hydrator->>MismatchCheck: Check for class mismatch
    MismatchCheck->>Child: Retrieve $cls (if present)
    MismatchCheck->>Child: Fallback to class attribute if $cls not present

Loading

Assessment against linked issues

Objective	Addressed	Explanation
Prevent hydration mismatch for class attribute in <Transition appear> with SSR (#13335)	✅

Poem

A class was once lost in the SSR mist,
Hydration would grumble, "Something's been missed!"
Now with a cache, mismatches are few—
The DOM and the client both stick like glue.
🐇✨
Hooray for the fix, from this coding rabbit to you!
"""

---

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📥 Commits

Reviewing files that changed from the base of the PR and between 26a579b and f5998fb.

📒 Files selected for processing (1)

• packages/runtime-core/__tests__/hydration.spec.ts (1 hunks)

🔇 Additional comments (1)

packages/runtime-core/__tests__/hydration.spec.ts (1)

1657-1678: Well-structured test for the transition appear hydration edge case.

This test effectively validates that elements with pre-existing classes maintain those classes when hydrated inside a Transition component with the appear prop. The test correctly ensures that both the original "foo" class and the transition-specific classes ("v-enter-from" and "v-enter-active") are preserved during hydration without any mismatch warnings.

✨ Finishing Touches

• 📝 Generate Docstrings

---

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[github-actions]

Size Report

Bundles

File	Size	Gzip	Brotli
runtime-dom.global.prod.js	101 kB (+33 B)	38.3 kB (+17 B)	34.5 kB (+16 B)
vue.global.prod.js	159 kB (+33 B)	58.5 kB (+15 B)	52.1 kB (+109 B)

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 (+33 B)	23 kB (+14 B)	20.9 kB (+14 B)
defineCustomElement	59.5 kB	22.8 kB	20.8 kB
overall	68.6 kB	26.4 kB	24.1 kB

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commit: f5998fb