A radial gradient on a box with no width or height is a circle across it

Review fix from aiongg (6db40bdb, the radial_ellipse part), ported to this
branch: a straight line's box has no ellipse, so its stroke's radial
gradient showed only the last stop once #823's gradient strokes land.

Co-authored-by: aiongg <aiongtaigi@gmail.com>
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

crates/doc/src/item.rs

@@ -44,7 +44,13 @@ impl Fill {
                 let r = designcraft_geom::Vec2::new(x1 - x0, (y1 - y0) / k).hypot();
                 (designcraft_geom::Point::new(x0, y0), r, r * k)
             }
-            None => (b.center(), b.width().max(1e-9) / 2.0, b.height().max(1e-9) / 2.0),
+            // A box with no width or height (a straight line's) has no ellipse: a circle across
+            // its length.
+            None if b.width() <= 1e-9 || b.height() <= 1e-9 => {
+                let r = b.width().max(b.height()).max(1e-3) / 2.0;
+                (b.center(), r, r)
+            }
+            None => (b.center(), b.width() / 2.0, b.height() / 2.0),
         }
     }
 

crates/engine/src/cmd/style.rs

@@ -1880,6 +1880,22 @@ mod radial_gradient_tests {
         let (c, side, top) = sides(&r.render_page(&d, &p.cache, 0, 1.0, false, &Default::default()).unwrap());
         assert!(c < 30 && side > 220 && top > 220, "pdf: centre {c}, side {side}, top {top}");
     }
+
+    #[test]
+    fn a_box_with_no_height_or_width_has_a_circle_across_its_length() {
+        let f = designcraft_doc::Fill::swatch("g");
+        let line = designcraft_geom::Rect::new(20.0, 60.0, 180.0, 60.0);
+        let (c, rx, ry) = f.radial_ellipse(line);
+        assert_eq!((c.x, c.y, rx, ry), (100.0, 60.0, 80.0, 80.0));
+        let (_, rx, ry) = f.radial_ellipse(designcraft_geom::Rect::new(10.0, 0.0, 10.0, 50.0));
+        assert_eq!((rx, ry), (25.0, 25.0));
+        // A point: a tiny circle, never a zero radius.
+        let (_, rx, ry) = f.radial_ellipse(designcraft_geom::Rect::new(5.0, 5.0, 5.0, 5.0));
+        assert!(rx > 0.0 && rx == ry);
+        // A box keeps its ellipse.
+        let (_, rx, ry) = f.radial_ellipse(designcraft_geom::Rect::new(0.0, 0.0, 100.0, 200.0));
+        assert_eq!((rx, ry), (50.0, 100.0));
+    }
 }
 
 #[cfg(test)]