fix(transition): fix KeepAlive with transition out-in mode behavior in production

[yangxiuxiu1115]

fix #12465

Summary by CodeRabbit

• Tests
  • Added a new test to verify "out-in" transition mode with KeepAlive and fallthrough attributes in production mode.
• Bug Fixes
  • Improved handling of component subtrees in transitions to ensure consistent behavior across development and production environments.

[github-actions]

Size Report

Bundles

File	Size	Gzip	Brotli
runtime-dom.global.prod.js	101 kB (+42 B)	38.2 kB (+11 B)	34.4 kB (-14 B)
vue.global.prod.js	159 kB (+42 B)	58.4 kB (+11 B)	51.9 kB (+34 B)

Usages

Name	Size	Gzip	Brotli
createApp (CAPI only)	46.5 kB	18.2 kB	16.6 kB
createApp	54.5 kB	21.2 kB	19.4 kB
createSSRApp	58.7 kB	22.9 kB	20.9 kB
defineCustomElement	59.3 kB	22.8 kB	20.8 kB
overall	68.6 kB (+42 B)	26.4 kB (+11 B)	24.1 kB (+49 B)

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commit: 237ff5f

[skirtles-code]

That branch of the code has a comment on it:

  // #7121 ensure get the child component subtree in case
  // it's been replaced during HMR

It would appear that the author of that section only intended for it to be used during HMR. If it's going to be used in production then that comment needs updating.

Did you investigate why that code was added just for HMR?

---

More generally, could you also please include more details in your PR descriptions? Presumably you spent a while debugging this before deciding what the appropriate fix should be. What did you find? Why do you think this is the correct fix? If you don't provide that information in the description then anyone reviewing the change is going to need to go through that debugging and exploration process all over again.

Reviewing a PR, even a small one like this, can take a reviewer several hours. We don't just need to check that the fix works, we need to assess whether it's actually the best fix for the underlying problem. Providing a brief explanation in the PR description of what you think the underlying problem is and why you think it's the right fix can save us a huge amount of time. It can also be really valuable months down the line when someone wants to understand why a particular change was made.

Thanks.

[yangxiuxiu1115]

Thank you very much for your reply, honestly I'm not very good at English, most of the time I rely on translation software, how to use English accurately might be really difficult for me, but that's my problem.

Return to PR, To be honest I did overlook that code comment you mentioned. But this is just my simple attempt, he may not be correct

debugging the issue is really tricky, one thing to note about this issue is that when class=“test” is removed from the transition, everything goes back to normal. This is due to the indirect effect of fallthroughAttrs, which clones a new vnode when it exists, which causes vnode.component.subTree and vnode.children[0] to get completely different values.

Then look at the difference between DEV and PROD, when there are fallthroughAttrs then vnode.component.subTree is the correct child vnode, so everything works fine in the DEV environment too!

NOTE:
The overall debugging idea can be started from the vnode, you can simply debug the number of updates on DEV and PROD, you can find that there is a missing update on PROD, and the last update is triggered by the afterLeave on the transition.

[Copilot]

Copilot reviewed 1 out of 1 changed files in this pull request and generated no suggestions.

[edison1105]

I think this PR should be correct. It is similar to #7121 where the root node has switched. We should always apply the transition to their component.subTree without distinguishing between DEV and PROD if the component is already mounted. The comments mentioned above need to be updated.

This needs a test case by the way.

[coderabbitai]

Walkthrough

A test was added to verify that the BaseTransition component with KeepAlive and fallthrough attributes behaves identically in both development and production modes. Additionally, the getInnerChild function in BaseTransition.ts was updated to ensure consistent handling of component subtrees, removing a development-only condition and updating related comments.

Changes

File(s)	Change Summary
packages/runtime-core/tests/components/BaseTransition.spec.ts	Added a test for "out-in" mode with KeepAlive and fallthrough attributes in production mode.
packages/runtime-core/src/components/BaseTransition.ts	Modified getInnerChild to remove the __DEV__ guard, ensuring consistent behavior between dev and prod.

Sequence Diagram(s)

sequenceDiagram
    participant TestRunner
    participant BaseTransition
    participant KeepAlive
    participant ComponentA
    participant ComponentB

    TestRunner->>BaseTransition: Mount with KeepAlive and fallthrough attrs (prod mode)
    BaseTransition->>KeepAlive: Render branch A (ComponentA)
    KeepAlive->>ComponentA: Render with class attribute
    TestRunner->>BaseTransition: Toggle to branch B
    BaseTransition->>ComponentA: Trigger leave hooks
    Note right of BaseTransition: onBeforeLeave/onLeave called
    BaseTransition->>KeepAlive: After leave, render branch B (ComponentB)
    KeepAlive->>ComponentB: Render with class attribute

Loading

Assessment against linked issues

Objective	Addressed	Explanation
Ensure consistent BaseTransition + KeepAlive + fallthrough attribute behavior in dev/prod (#12465)	✅
Remove dev-only logic in getInnerChild for consistent subtree handling (#12465)	✅

Poem

A rabbit hopped through code so bright,
Ensuring transitions worked just right.
Dev and prod now act the same,
No more bugs to cause us shame!
With tests in place, we leap with glee—
Consistency for all to see! 🐇✨

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📥 Commits

Reviewing files that changed from the base of the PR and between 258f78b and 237ff5f.

📒 Files selected for processing (2)

• packages/runtime-core/__tests__/components/BaseTransition.spec.ts (1 hunks)
• packages/runtime-core/src/components/BaseTransition.ts (1 hunks)

⏰ Context from checks skipped due to timeout of 90000ms (1)

• GitHub Check: test / e2e-test

🔇 Additional comments (2)

packages/runtime-core/src/components/BaseTransition.ts (1)

504-505: Ensure consistent component subtree handling in all environments

This change fixes the inconsistency between development and production builds by always using the component's subtree if the component exists, regardless of build environment. The updated comment correctly documents both issue references (#7121, #12465) and clarifies the purpose of this code.

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

1202-1247: Great test coverage for the production mode fix

This test correctly verifies the fix by:

1. Temporarily setting __DEV__ = false to simulate production environment
2. Testing the specific scenario with KeepAlive, "out-in" transition mode, and fallthrough attributes
3. Verifying that the DOM elements maintain the correct class attributes during transitions
4. Confirming the expected hook call sequence

The test thoroughly validates that the fix in BaseTransition.ts works as expected in production builds.

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[edison1105]

/ecosystem-ci run

[vue-bot]

📝 Ran ecosystem CI: Open

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