showPill property

Whether the compact search pill exists on the bar.

Defaults to true — the pill is present in both states, the original layout. false removes it entirely: the tab pill takes over the reclaimed width, and the pill is genuinely absent rather than invisible — it contributes no glass shape to the blend layer, so nothing fuses with the tab pill's trailing edge. In iPhone Duo's vertical bar strip the capsule likewise ends at the last tab, with no search slot.

The value may change at runtime: the pill spring-scales in and out in place at its slot, on the same spring the pill morphs use. That is the building block for app-driven policies — a bar that wants the pill only while search is active simply flips this together with isSearchActive.