Function prev_grapheme_boundary 

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

Previous extended grapheme cluster boundary before from.

Drives CursorMotion::CharLeft, so Left / Shift+Left / forward-word fallbacks all step whole clusters (Qt previousCursorPosition, Slint GraphemeCursor::prev_boundary). Backspace does NOT use this; see prev_code_point_boundary.