Function prev_code_point_boundary 

pub fn prev_code_point_boundary(s: &str, from: usize) -> usize

Previous code point boundary before from.

Backspace only. Qt’s backspace() steps one code unit and Slint keeps a dedicated PreviousCharacter direction for the same reason: peeling a combining mark off the base character backwards is the expected editing behavior. Keep this asymmetry with prev_grapheme_boundary.