The pill never appeared because:
1. SwiftUI .frame(width:0, height:0) when idle poisoned fittingSize
2. AppKit constraints locked at 0x0 prevented expansion on state change
3. fittingSize always returned 0 due to active 0x0 constraints
Fix: Remove zero-frame from SwiftUI (always render at natural size,
use opacity only). Deactivate constraints before measuring fittingSize
so they don't clamp the measurement. Pass visibility to sizeToolbarItem
to set constraints to zero when idle or natural size when active.
NSHostingView with frame (0,0) won't layout SwiftUI content, so
fittingSize always returns 0 when transitioning from EmptyView.
Instead, always render the pill button and use opacity/frame(0)
to hide when idle. This ensures the hosting view always has content
to measure.
