A rapid second tap on Next (easy to trigger with Apple Pencil) called
advance() twice before the view tree re-rendered, skipping a card.
Share a single isAdvancing flag between advance() and a new goBack()
helper, disable both buttons while the flag is set, and clear it after
350 ms so queued Pencil events are absorbed.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
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