Stable key. Re-registering the same id replaces the prior panel — re-mounting a companion refreshes, doesn't stack.
Visible tab label.
Optional
Called when the user clicks × on the tab. Typically () => unmount() — the dock then detach()es so a double-detach is a no-op.
() => unmount()
detach()
Stable key. Re-registering the same id replaces the prior panel — re-mounting a companion refreshes, doesn't stack.