fix: use correct cursor scroll position for wide (CJK) characters (#10)

Use unicode display width instead of char count when computing the
horizontal scroll offset so the cursor stays in view when lines contain
wide characters such as CJK ideographs.

Ported from rhysd/tui-textarea#94.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: pm100

src/widget.rs

@@ -7,6 +7,7 @@ use ratatui_core::widgets::Widget;
 use ratatui_widgets::paragraph::Paragraph;
 use std::cmp;
 use std::sync::atomic::{AtomicU64, Ordering};
+use unicode_width::UnicodeWidthChar;
 
 // &mut 'a (u16, u16, u16, u16) is not available since `render` method takes immutable reference of TextArea
 // instance. In the case, the TextArea instance cannot be accessed from any other objects since it is mutablly
@@ -111,7 +112,13 @@ impl<'a> TextArea<'a> {
     }
 
     fn scroll_top_col(&self, prev_top: u16, width: u16) -> u16 {
-        let mut cursor = self.cursor().1 as u16;
+        let (row, col) = self.cursor();
+        // Adjust the cursor position to account for the display width of non-ASCII characters.
+        let mut cursor = self.lines()[row]
+            .chars()
+            .take(col)
+            .map(|c| c.width().unwrap_or(0))
+            .sum::<usize>() as u16;
         // Adjust the cursor position due to the width of line number.
         if self.line_number_style().is_some() {
             let lnum = num_digits(self.lines().len()) as u16 + 2; // `+ 2` for margins