Apply same correction for mid_lines() based chars as well

Also update changelog

docs/changelog.rst

@@ -165,6 +165,11 @@ consumption to do the same tasks.
 Detailed list of changes
 -------------------------------------
 
+0.47.0 [future]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+- Fix thickness of diagonal lines in box drawing characters not the same as horizontal/vertical lines (:iss:`9719`)
+
 0.46.2 [2026-03-21]
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 

kitty/decorations.c

@@ -1269,7 +1269,7 @@ mid_lines(Canvas *self, uint level, ...) {
             case BOTTOM_LEFT: p1 = l; p2 = b; break;
             case BOTTOM_RIGHT: p1 = r; p2 = b; break;
         }
-        thick_line(self, th, p1, p2);
+        thick_line(self, diagonal_thickness(th, p1, p2), p1, p2);
     }
     va_end(args);
 }