Function adjust_single_index_for_change 

pub fn adjust_single_index_for_change(
    idx: usize,
    change: &DataChange,
) -> Option<usize>

Map a single index anchor (not a selection set) through a DataChange, or None if the row the anchor pointed at no longer exists (it was removed, or the whole list was reset).

This is the same shift semantics as map_index_after_move / SelectionModel::adjust_for_* / CheckedModel::adjust_for_*, specialized for a bare Option<usize> anchor that has no “membership” to prune — e.g. ListView’s keyboard-focus index. Used so a single-anchor consumer doesn’t have to re-derive insert/remove/move shift logic by hand.

• ItemsInserted: the anchor shifts up by the inserted count if it sat at or after the insertion point, otherwise it’s untouched.
• ItemsRemoved: the anchor shifts down past the removed range; if the anchor itself pointed into the removed range, it is dropped (None) — the row it followed is gone.
• ItemsMoved: delegates to map_index_after_move (the anchor follows its row, or shifts around the moved block like everyone else).
• ItemUpdated / WindowLoaded: no structural shift — the anchor is unchanged.
• Reset: the anchor is dropped (None) — nothing about the old indexing survives a wholesale replacement.