[stipsan]

Smooth scrolling now completes reliably for zero-duration animations, disconnected targets, and custom easing functions, and preserves custom behavior return values. Final animation coordinates always reach the requested destination.

• Forward container/options to the base ponyfill and improve types for smooth action-array completion and standard native completion.
• Resolve disconnected smooth calls to an empty action array and avoid NaN for zero duration.
• Keep custom callback results unchanged, including no-op calls, and add regressions for completion and container forwarding.
• Accept the upcoming base ponyfill major with ^3.1.0 || ^4.0.0 and include a major changeset.

Migration: the default smooth completion is explicitly typed as Promise<SmoothScrollAction[]>; standard behavior inherits the base ponyfill's native completion contract when using its new major. Custom callbacks may run with an empty action list. Native ScrollResult and the add-on's animation action array are distinct results.

Validation: Node 26 typecheck and production build; all 12 tests passed. Coordinated integration with the updated local engine and base ponyfill also passed.

Release dependency: publish the engine, then the base ponyfill major, then this add-on major. The lockfile retains the published base ponyfill until that release exists; refresh it after publication.

Related PRs in release order:

1. feat: align scroll geometry with current CSSOM behavior compute-scroll-into-view#930
2. feat!: support native scroll completion and consistent defaults scroll-into-view-if-needed#1226
3. fix!: make smooth scrolling completion reliable #1075
4. docs: refresh scrolling standards and compatibility guidance scroll-into-view.dev#1127

Want tembo to make any changes? Add a comment with @tembo and i'll get back to work!