Stacked bars each reserve the system bar

A bar that can be used two ways takes its insets as a parameter, defaulted to the framework value:

@Composable
fun SelectionTopAppBar(
    state: SelectionState,
    /* … */
    // Defaults to the status-bar inset, like any top bar. Pass WindowInsets(0) when this bar is
    // STACKED below something that already consumed that inset.
    windowInsets: WindowInsets = TopAppBarDefaults.windowInsets,
) {
    TopAppBar(windowInsets = windowInsets, /* … */)
}