Reading issue
sveltejs / svelte · Issue No. 18840
A recoverable hydration mismatch is intercepted by <svelte:boundary> when hydrating a SvelteKit 3 app. The boundary treats Svelte's internal hydration failure signal as an application render error, so SvelteKit renders its +error.svelte page instead of allowing top-level hydrate() recovery.
Originally reported in sveltejs/kit#17151.
In the client error-boundary dispatch, rethrow the private HYDRATION_ERROR sentinel before traversing boundaries or applying transformError, preserving its identity for the catch in hydrate().
Add regression coverage asserting that malformed {@html} beneath a boundary warns and recovers without rendering the fallback, while genuine hydration-time component exceptions remain catchable.
Relay reads this issue against the repository's contribution signals: the files it is likely to touch, how the maintainers triage work this size, and what the first contribution would exercise.
The full analysis for this issue is still being assembled. Until then, the description above and the thread on GitHub are the most reliable context.