MutationObserver injected before page JS boots watches the stepper
island for content removal during hydration. Detects 55 node removals
— the island hydration destroys SSR DOM and rebuilds it, causing a
visible flash.
Test correctly fails: "No clobber: 55 removals"
This is the root cause of the flash — island hydration needs to
preserve SSR content instead of replacing it.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
giles
3329512bf8