fix(suspense): don't treat the leaving branch as the fallback while its mount is pending by zingiberoff · Pull Request #15333 · vuejs/core

zingiberoff · 2026-08-20T22:11:13Z

When <Suspense timeout> is wrapped in <Transition mode="out-in">, the fallback mount is deferred to the leaving branch's afterLeave. A parent update arriving in that window hit patchSuspense's isInFallback paths, which patched activeBranch (still the leaving content) into the fallback and made it the active branch without clearing isFallbackMountPending. resolve() then skipped unmounting the active branch (the flag was still set) yet attached the content insertion to its afterLeave — which never fires for a branch that is never unmounted, so the resolved content was permanently dropped and the fallback stayed on screen.

This PR skips the fallback patch while isFallbackMountPending is true: the fallback is not the active branch yet, and mountFallback will bring it in on afterLeave as intended.

Regression introduced by 908c6ad (#9392), first released in v3.5.31; 3.5.30 is unaffected.

Verification:

deterministic reproduction in Suspense timeout + Transition out-in: parent update during the fallback swap permanently drops the resolved branch #15332 (fails on the first run on 3.5.31+, passes on 3.5.30);
added e2e regression test parent update during timed-out fallback swap with out-in transition — fails without the fix, passes with it;
full Transition e2e suite passes (57/57, including the fix(suspense): avoid unmount activeBranch twice if wrapped in transition #9392 test, so App crashes when using router-view, transition, suspense and teleport #7966 is not reintroduced); Suspense + BaseTransition unit suites pass (97/97).

Summary by CodeRabbit

Bug Fixes
- Fixed Suspense fallback transitions with out-in animations.
- Prevented resolved content from being replaced prematurely during fallback swaps.
- Ensured fallback content is removed correctly after the async branch resolves.
- Updated deferred fallback rendering to reflect the latest parent content.
Tests
- Added regression coverage for parent updates during animated Suspense fallback transitions.
- Added coverage for fallback updates before an asynchronous branch resolves.

…ts mount is pending When <Suspense timeout> is wrapped in <Transition mode="out-in">, the fallback mount is deferred to the leaving branch's afterLeave. A parent update arriving in that window hit patchSuspense's isInFallback paths, which patched activeBranch (still the leaving content) into the fallback and made it the active branch without clearing isFallbackMountPending. resolve() then skipped unmounting the active branch (the flag was still set) yet attached the content insertion to its afterLeave — which never fires for a branch that is never unmounted, so the resolved content was permanently dropped and the fallback stayed on screen. Skip the fallback patch while isFallbackMountPending is true: the fallback is not the active branch yet, and mountFallback will bring it in on afterLeave as intended. Regression introduced by 908c6ad (vuejs#9392) in v3.5.31. close vuejs#15332 Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

coderabbitai · 2026-08-20T22:11:33Z

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro Plus

Run ID: 1ad7c1b4-86f5-4263-b05b-cdc89702f4ad

📥 Commits

Reviewing files that changed from the base of the PR and between 52b1cb2 and 1d63ba0.

📒 Files selected for processing (2)

packages/runtime-core/src/components/Suspense.ts
packages/vue/__tests__/e2e/Transition.spec.ts

Included review availability: Your plan provides up to 10 included reviews per hour; 8 remain after this review.

📝 Walkthrough

Walkthrough

Suspense now skips fallback updates during deferred out-in transitions and mounts the latest fallback vnode after the leave transition. Two end-to-end tests cover parent updates during fallback swapping and deferred fallback mounting.

Changes

Suspense transition fix

Layer / File(s)	Summary
Deferred fallback patching `packages/runtime-core/src/components/Suspense.ts`	Suspense skips same-root and branch-toggle fallback patches while fallback mounting is deferred. Deferred mounting uses the latest fallback vnode from the suspense vnode.
Out-in transition regression coverage `packages/vue/__tests__/e2e/Transition.spec.ts`	Two end-to-end tests validate parent updates during fallback swapping and deferred fallback mounting before the async branch resolves.

Estimated code review effort: 3 (Moderate) | ~20 minutes

Merge Risk: ⚪ Minimal · up to 1d63b

This localized Suspense fix addresses the pending fallback transition behavior and includes regression coverage; no actionable merge-blocking risk remains beyond normal checks and review.

Suggested reviewers: edison1105, yangxiuxiu1115

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 2 functions across 2 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Linked Issues check	✅ Passed	The changes address issue `#15332` by preventing premature fallback patching and preserving deferred fallback mounting during out-in transitions.
Out of Scope Changes check	✅ Passed	The runtime fix and regression tests directly support the requirements in issue `#15332`, with no unrelated changes identified.
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly describes the Suspense fix for deferred fallback mounting during an out-in transition.

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Actionable comments posted: 1

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Inline comments:
In `@packages/runtime-core/src/components/Suspense.ts`:
- Line 275: Update mountFallback to read the latest fallback VNode from
suspense.vnode.ssFallback at mount time, and use that VNode consistently for
both patch() and setActiveBranch(). Apply the same correction to the related
deferred-leave path guarded by isFallbackMountPending, and add coverage for
parent-updated fallback content resolving after leave completes.

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ℹ️ Review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro Plus

Run ID: 3ac92147-d4b4-462a-be49-cc6c442d5bbc

📥 Commits

Reviewing files that changed from the base of the PR and between a2b40db and 52b1cb2.

📒 Files selected for processing (2)

packages/runtime-core/src/components/Suspense.ts
packages/vue/__tests__/e2e/Transition.spec.ts

Included review availability: Your plan provides up to 10 included reviews per hour; 9 remain after this review.

A parent update arriving while the fallback mount is deferred to the leaving branch's afterLeave can produce a newer fallback vnode; its patch is skipped during that window, so mountFallback must read the latest ssFallback from suspense.vnode instead of the captured one. Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

pkg-pr-new · 2026-08-21T00:36:17Z

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commit: 1d63ba0

github-actions · 2026-08-21T00:46:57Z

Size Report

Bundles

File	Size	Gzip	Brotli
runtime-dom.global.prod.js	108 kB (+85 B)	40.8 kB (+26 B)	36.6 kB (+37 B)
vue.global.prod.js	167 kB (+85 B)	61 kB (+21 B)	54.2 kB (+1 B)

Usages

Name	Size	Gzip	Brotli
createApp (CAPI only)	49.7 kB	19.3 kB	17.7 kB
createApp	57.8 kB	22.3 kB	20.4 kB
createSSRApp	62.5 kB	24.3 kB	22.1 kB
defineCustomElement	64.1 kB	24.3 kB	22.1 kB
overall	72.2 kB (+85 B)	27.6 kB (+23 B)	25.1 kB (+29 B)

edison1105 · 2026-08-21T00:55:08Z

/ecosystem-ci run

vue-bot · 2026-08-21T01:14:20Z

📝 Ran ecosystem CI: Open

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

coderabbitai Bot reviewed Aug 20, 2026

View reviewed changes

Comment thread packages/runtime-core/src/components/Suspense.ts

zingiberoff changed the title ~~fix(suspense): don't treat the leaving branch as the fallback while its mount is pending~~ fix(suspense): don't treat the leaving branch as the fallback while its mount is pending (fix #15332) Aug 20, 2026

edison1105 changed the title ~~fix(suspense): don't treat the leaving branch as the fallback while its mount is pending (fix #15332)~~ fix(suspense): don't treat the leaving branch as the fallback while its mount is pending Aug 21, 2026

edison1105 added ready to merge The PR is ready to be merged. scope: suspense 🔨 p3-minor-bug Priority 3: this fixes a bug, but is an edge case that only affects very specific usage. labels Aug 21, 2026

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fix(suspense): don't treat the leaving branch as the fallback while its mount is pending - #15333

fix(suspense): don't treat the leaving branch as the fallback while its mount is pending#15333
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