presentationPlacement(.leading) sheet won't go half width on iPhone Duo when fully unfolded

My map app uses a persistent bottom sheet over a full-screen Map. On the iPhone Duo I want the sheet pinned to the leading side at about half the screen width. When the device is half-open, .presentationPlacement(.leading) works and the sheet sits on the left . When it's fully unfolded, the sheet stretches much wider and ignores the placement . The Flighty app manages to show a half-width sheet on the Duo even when it's fully unfolded, so it seems possible. Does anyone know how they ...