Embedded signing emits hydration errors and falsely reports failure after completion

Description

On Documenso Cloud's Platform plan, the whitelabel signing embed repeatedly emits React hydration errors and can show a failed-submission screen even though the envelope completed successfully.

Observed on 2026-08-01 with a 48-page PDF provisioned through API v2, distributionMethod: NONE, vendor email delivery disabled, and the signer mounted at /embed/sign/<signing-token>.

Steps to reproduce

1. Provision a pending envelope through API v2 and mount its recipient signing token in the whitelabel iframe embed.
2. Complete the visible required fields.
3. Select Complete, then confirm Sign.

Actual behavior

• Each healthy embed mount emits repeated minified React error #418 and one #423 from assets/index-DvCfNlkf.js.
• After the final confirmation, the frame displayed Something went wrong and said it was unable to submit the document. Retrying showed the same message.
• An owned API read nevertheless reported the envelope as COMPLETED.
• Reloading the embed then showed Document Completed! for the signer.
• The only other console warnings observed were a Konva no-parent warning and a missing dialog-description warning.

In a separate healthy PENDING mount, a parent listener restricted to the exact Documenso origin and frame source observed no postMessage payload containing a string action during the first 15 seconds after the frame had rendered. Please confirm the supported embed event contract for ready/completed/rejected/error notifications, if one exists.

No workaround was applied to the vendor frame. The host now treats the iframe's browser load event as its local mount-ready signal.

Expected behavior

• A successful signing completion should show the successful completion state rather than a failure that invites a duplicate retry.
• The embed should hydrate without React mismatch/recovery errors.
• The supported cross-origin event contract, if any, should be documented or otherwise discoverable for Platform embed consumers.

Additional context

• Signing did complete despite the false failure screen.
• The embed rendered responsively and without Documenso branding.
• This report intentionally omits signing tokens, envelope identifiers, and recipient addresses.