update `x_offset` calculation in Buffer::set_string_truncated (#3839)

when `truncate_start` is `true`, the `x_offset` is now properly updated
according to the width of the content or the truncated length.

helix-tui/src/buffer.rs

@@ -360,14 +360,14 @@ impl Buffer {
             let mut start_index = self.index_of(x, y);
             let mut index = self.index_of(max_offset as u16, y);
 
-            let total_width = string.width();
+            let content_width = string.width();
-            let truncated = total_width > width;
+            let truncated = content_width > width;
             if ellipsis && truncated {
                 self.content[start_index].set_symbol("…");
                 start_index += 1;
             }
             if !truncated {
-                index -= width - total_width;
+                index -= width - content_width;
             }
             for (byte_offset, s) in graphemes.rev() {
                 let width = s.width();
@@ -384,6 +384,7 @@ impl Buffer {
                     self.content[i].reset();
                 }
                 index -= width;
+                x_offset += width;
             }
         }
         (x_offset as u16, y)