vi: Implement :set number line display

Display line numbers with "%6d  " format (8 chars) when number option
is enabled. Adjusts available columns and cursor positioning accordingly.

Fixes #530

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: jgarzik

editors/tests/pty/mod.rs

@@ -272,3 +272,57 @@ fn test_pty_vi_utf8_display() {
     let contents = std::fs::read_to_string(&file_path).unwrap();
     assert_eq!(contents, "Привет мир\n");
 }
+
+/// Test: `:set number` displays line numbers without panic.
+/// Regression test for issue #530.
+#[test]
+fn test_pty_vi_set_number() {
+    let td = tempdir().unwrap();
+    let file_path = td.path().join("test_number.txt");
+    std::fs::write(&file_path, "line1\nline2\nline3\n").unwrap();
+
+    let pty_system = native_pty_system();
+    let pair = pty_system
+        .openpty(PtySize {
+            rows: 25,
+            cols: 80,
+            pixel_width: 0,
+            pixel_height: 0,
+        })
+        .unwrap();
+
+    let mut cmd = CommandBuilder::new(env!("CARGO_BIN_EXE_vi"));
+    cmd.arg(&file_path);
+    cmd.env("TERM", "vt100");
+
+    let mut child = pair.slave.spawn_command(cmd).unwrap();
+    drop(pair.slave);
+
+    let reader = pair.master.try_clone_reader().unwrap();
+    let _reader_thread = spawn_reader_drain(reader);
+    let mut writer = pair.master.take_writer().unwrap();
+
+    // Wait for vi startup
+    thread::sleep(Duration::from_millis(500));
+
+    // Enable line numbers
+    write_keys(&mut writer, ":set number\r");
+    thread::sleep(Duration::from_millis(200));
+
+    // Move cursor to verify positioning works with line numbers
+    write_keys(&mut writer, "jjk");
+    thread::sleep(Duration::from_millis(100));
+
+    // Disable line numbers
+    write_keys(&mut writer, ":set nonumber\r");
+    thread::sleep(Duration::from_millis(100));
+
+    // Quit without saving
+    write_keys(&mut writer, ":q!\r");
+
+    wait_with_timeout(&mut child, Duration::from_secs(5));
+
+    // File should be unchanged
+    let contents = std::fs::read_to_string(&file_path).unwrap();
+    assert_eq!(contents, "line1\nline2\nline3\n");
+}

editors/vi/editor.rs

@@ -3013,6 +3013,8 @@ impl Editor {
 
     /// Refresh the screen.
     fn refresh_screen(&mut self) -> Result<()> {
+        const LINE_NUMBER_WIDTH: usize = 8; // "%6d  " format
+
         let size = self.terminal.size();
         self.screen.resize(size);
         self.screen
@@ -3032,8 +3034,17 @@ impl Editor {
 
             if line_num <= self.buffer.line_count() {
                 if let Some(line) = self.buffer.line(line_num) {
+                    // Line number prefix when option set
+                    if self.options.number {
+                        self.terminal.write_str(&format!("{:6}  ", line_num))?;
+                    }
+                    let avail_cols = if self.options.number {
+                        (size.cols as usize).saturating_sub(LINE_NUMBER_WIDTH)
+                    } else {
+                        size.cols as usize
+                    };
                     // Expand tabs and truncate (expand_line already caps at max_cols)
-                    let content = self.screen.expand_line(line.content(), size.cols as usize);
+                    let content = self.screen.expand_line(line.content(), avail_cols);
                     self.terminal.write_str(&content)?;
                 }
             } else {
@@ -3058,6 +3069,12 @@ impl Editor {
             cursor.column,
         ) as u16
             + 1;
+        // Add line number offset
+        let display_col = if self.options.number {
+            display_col + LINE_NUMBER_WIDTH as u16
+        } else {
+            display_col
+        };
 
         if self.mode == Mode::Ex {
             // Cursor at command line