fix: sharp rectangle corners in the export

src/editor/export.rs

@@ -109,6 +109,19 @@ mod tests {
         );
     }
 
+    #[test]
+    fn rectangles_have_sharp_corners() {
+        let base = RgbaImage::from_pixel(60, 40, Rgba([255, 255, 255, 255]));
+        let rect = Shape {
+            tool: Tool::Rectangle,
+            style: Style { color: [255, 0, 0], width: 8.0 },
+            points: vec![(10.0, 10.0), (50.0, 30.0)],
+        };
+        let out = render(&base, &[rect]);
+        // A round join would leave the outer corner (6..7, 6..7) white.
+        assert_eq!(out.get_pixel(6, 6).0, [255, 0, 0, 255]);
+    }
+
     #[test]
     fn highlighter_blends_at_40_percent() {
         let base = RgbaImage::from_pixel(100, 40, Rgba([0, 0, 255, 255]));

src/editor/shape.rs

@@ -154,7 +154,7 @@ pub enum Prim {
         width: f32,
         color: [u8; 4],
         closed: bool,
-        /// Round caps and joins; false means butt caps.
+        /// Round caps and joins; false means butt caps and miter joins.
         round: bool,
     },
     /// A convex polygon.
@@ -178,7 +178,14 @@ pub fn geometry(shape: &Shape) -> Vec<Prim> {
         Tool::Arrow => arrow(pts[0], pts[1], w, opaque),
         Tool::Rectangle => {
             let ((x0, y0), (x1, y1)) = (pts[0], pts[1]);
-            vec![stroke(vec![(x0, y0), (x1, y0), (x1, y1), (x0, y1)], true)]
+            // Sharp (miter) corners, as egui draws them while dragging.
+            vec![Prim::Stroke {
+                points: vec![(x0, y0), (x1, y0), (x1, y1), (x0, y1)],
+                width: w,
+                color: opaque,
+                closed: true,
+                round: false,
+            }]
         }
         Tool::Ellipse => {
             let ((x0, y0), (x1, y1)) = (pts[0], pts[1]);