Fix max box width calculation

src/codebox.ts

@@ -747,7 +747,8 @@ export class CodeBox {
 
         let maxWidth = 0;
         for (const row of this.box) {
-            if (maxWidth > row.length) { maxWidth = row.length; }
+            if (row.length > maxWidth) { maxWidth = row.length; }
         }
+        this.maxBoxWidth = maxWidth;
     }
 };