navigationTitle/principal items in the NavigationStack toolbar silently fail to
refresh on scroll-driven state changes (visibleMonth), so the month label
vanished on month change and only returned after an unrelated rebuild. The glass
variant now uses a custom top bar with the same inline Text title as the flat
variant (proven to update), styled with glassEffect (iOS 26) / ultraThinMaterial.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
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