Function button_group 

pub fn button_group<State: 'static, F: Fn(&mut State, usize) + 'static>(
    labels: impl IntoIterator<Item = impl Into<String>>,
    selected: usize,
    on_select: F,
) -> ButtonGroupView<State>

Create a single-select connected group with the given member labels, the currently-selected index, and an on_select callback fired (with the tapped member’s index) on release inside a member.

A selected index outside 0..labels.len() simply highlights no member (there is no panic); feed a valid index to show a selection.

This is the module’s pre-rework constructor and keeps its pre-rework meaning: ButtonGroupType::Connected, and an on_select that always reports the tapped index (never a deselect None). Reach for button_group_actions for the full port — icons, multi-select, densities, direction, and overflow — whose defaults follow the reference’s (Standard) instead.