diff --git a/docs/changelog.rst b/docs/changelog.rst
index d5285bb1a..87ce49374 100644
--- a/docs/changelog.rst
+++ b/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]
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
diff --git a/kitty/decorations.c b/kitty/decorations.c
index 887ab5212..e85c7aa56 100644
--- a/kitty/decorations.c
+++ b/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);
 }