chore(runtime-vapor): render fallback at the deepest invalid fragment

[edison1105]

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Size Report

Bundles

File	Size	Gzip	Brotli
compiler-dom.global.prod.js	85.2 kB	29.9 kB	26.4 kB
runtime-dom.global.prod.js	108 kB	40.5 kB	36.5 kB
vue.global.prod.js	166 kB	60.5 kB	53.9 kB

Usages

Name	Size	Gzip	Brotli
createApp (CAPI only)	48.2 kB	18.8 kB	17.2 kB
createApp	57.2 kB	22 kB	20.1 kB
createApp + vaporInteropPlugin	95.9 kB (-4 B)	35.4 kB (+6 B)	32 kB (+34 B)
createVaporApp	40.9 kB (-4 B)	15.2 kB (+13 B)	13.8 kB (+17 B)
createSSRApp	61.6 kB	23.8 kB	21.7 kB
defineCustomElement	63.3 kB	23.8 kB	21.7 kB
overall	71.9 kB	27.2 kB	24.8 kB

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commit: 6b8693d

[Copilot]

Pull request overview

This PR refactors the fragment fallback processing logic to prevent duplicate processing and improve invalid fragment detection through a more explicit, callback-based approach.

Key Changes:

• Introduces a processedFallbacks WeakMap to track which fallbacks have been applied to fragments, preventing redundant chaining when the same fallback is reused
• Replaces the findInvalidFragment recursive function with inline detection using a callback pattern that identifies the deepest invalid fragment during fallback traversal
• Adds an optional onFragment callback parameter to setFragmentFallback to enable side effects during fragment tree traversal

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