textbox.rs: Respect unicode width

Reference:
  ulyssa/iamb#293

Author: simnalamburt

Cargo.lock

@@ -19,6 +19,7 @@ modalkit = { workspace = true }
 ratatui = { version = "0.26" }
 regex = { workspace = true }
 serde = { version = "^1.0", features = ["derive"] }
+unicode-width = "0.2.0"
 
 [dev-dependencies]
 rand = { workspace = true }

crates/modalkit-ratatui/Cargo.toml

@@ -58,6 +58,8 @@ use modalkit::editing::{
 use modalkit::errors::{EditError, EditResult, UIResult};
 use modalkit::prelude::*;
 
+use unicode_width::UnicodeWidthStr;
+
 use super::{ScrollActions, TerminalCursor, WindowOps};
 
 /// Line annotation shown in the left gutter.
@@ -859,13 +861,18 @@ where
                 }
             }
 
-            let _ = buf.set_stringn(x, y, s, width, self.style);
-
             if cursor_line {
-                let coff = (cursor.x - start) as u16;
+                let coff = s[..s
+                    .char_indices()
+                    .map(|(i, _)| i)
+                    .nth(cursor.x.saturating_sub(start))
+                    .unwrap_or(s.len())].width_cjk() as u16;
+
                 state.term_cursor = (x + coff, y);
             }
 
+            let _ = buf.set_stringn(x, y, s, width, self.style);
+
             self._highlight_followers(line, start, end, (x, y), &finfo, buf);
             self._highlight_line(line, start, end, (x, y), &hinfo, buf);
 
@@ -1002,13 +1009,19 @@ where
 
             let s = s.to_string();
             let w = (right - x) as usize;
-            let (xres, _) = buf.set_stringn(x, y, s, w, self.style);
 
             if cursor_line {
-                let coff = cursor.x.saturating_sub(start) as u16;
+                let coff = s[..s
+                    .char_indices()
+                    .map(|(i, _)| i)
+                    .nth(cursor.x.saturating_sub(start))
+                    .unwrap_or(s.len())].width_cjk() as u16;
+
                 state.term_cursor = (x + coff, y);
             }
 
+            let (xres, _) = buf.set_stringn(x, y, s, w, self.style);
+
             self._highlight_followers(line, start, end, (x, y), &finfo, buf);
             self._highlight_line(line, start, end, (x, y), &hinfo, buf);