Scroll to the first error

handleSubmit(context, onValid) reveals the first invalid field when it is mounted. In a lazy list, an off-screen row may not have an anchor yet, so first scroll to the row's section and then ask the registry to reveal it.

Two-phase reveal#

1. Coarse scroll: map the first visible error key to its section using the row's clientId, then scroll until that section is built. In a sliver list, the target section's RenderSliver.constraints.precedingScrollExtent gives its logical offset. Walk the viewport toward that offset until the target has been laid out.
2. Fine reveal: after the next frame, call KeyedFieldRegistry.revealFirst to use the field anchor for precise positioning and focus.

Keep section keys by row clientId, not by index, so the mapping survives list reorder.

// Pseudocode: sectionForKey and jumpToSection are app-specific. For a lazy
// sliver, jumpToSection resolves the section's precedingScrollExtent and
// advances the scroll position until that section is laid out.
Future<void> revealFirstError(BuildContext context) async {
  final keys = form.visibleErrorKeys.toList();
  if (keys.isEmpty) return;

  await jumpToSection(sectionForKey(keys.first));
  if (!context.mounted) return;
  await WidgetsBinding.instance.endOfFrame;
  if (!context.mounted) return;

  KeyedForm.registryOf<TourSchema>(context).revealFirst(
    form.visibleErrorKeys,
    alignment: 0.5,
  );
}

The coarse pass is needed because an unbuilt row has no field anchor. Once the target is built, wait for WidgetsBinding.instance.endOfFrame before asking the registry to reveal and focus the exact field.

For forms without lazy sections, use handleSubmit directly; it handles the reveal and focus flow for mounted fields.