Use unicode-width

Cargo.lock

@@ -1018,6 +1018,7 @@ dependencies = [
  "tracing-appender",
  "tracing-panic",
  "tracing-subscriber",
+ "unicode-width",
 ]
 
 [[package]]

Cargo.toml

@@ -20,3 +20,4 @@ tracing = "0.1"
 tracing-appender = "0.2"
 tracing-panic = "0.1"
 tracing-subscriber = "0.3"
+unicode-width = "0.2.2"

src/tui/render.rs

@@ -116,7 +116,12 @@ impl TerminalState {
                     .map(|g| g.width(total_lines) as u16)
                     .sum();
                 let y = cursor.y() as usize;
-                let screen_x = (cursor.x() as usize).min(display_buffer.line_len(y)) as u16;
+                let line = display_buffer
+                    .visible_lines(y, y + 1)
+                    .into_iter()
+                    .next()
+                    .unwrap_or_default();
+                let screen_x = char_idx_to_display_col(&line, cursor.x() as usize) as u16;
                 frame.set_cursor_position((
                     rect.x + left_gutter_width + screen_x,
                     rect.y + cursor.y() as u16,
@@ -283,6 +288,17 @@ impl TerminalState {
     }
 }
 
+/// Convert a char index into a terminal display column by summing the display
+/// width of each preceding character. Wide characters (e.g. CJK, hiragana)
+/// count as 2 columns; narrow chars count as 1.
+fn char_idx_to_display_col(line: &str, char_idx: usize) -> usize {
+    use unicode_width::UnicodeWidthChar;
+    line.chars()
+        .take(char_idx)
+        .map(|c| c.width().unwrap_or(0))
+        .sum()
+}
+
 fn process_line(input: &str, expand_tab: usize) -> String {
     input
         .chars()