Tests: Intertwine keeps a third member in front of a raised overlap's margin

A red, blue and green rectangle are made into an Intertwine group, and
red is brought over blue where they overlap. The green sliver meets that
overlap's edge over red only. The test checks that green still draws in
front of red within the 2 pt margin, that red's raised area takes in the
margin where only red paints, and that it leaves out the part where
green paints too.

crates/engine/src/tests/intertwine.rs

@@ -104,6 +104,45 @@ fn the_object_brought_over_shows_no_seam_of_the_one_below() {
     assert!(worst < 8, "blue shows through ring A at the top crossing (blue {worst})");
 }
 
+fn green(r: &Rendered, x: u32, y: u32) -> bool {
+    let p = r.pixel(x, y);
+    p[3] > 200 && p[1] > 200 && p[0] < 60 && p[2] < 60
+}
+
+/// The margin an object brought over another is raised in past their overlap reaches only where
+/// that object alone paints: a third member that meets the overlap's edge stays in front of it
+/// there. Red A (20..80 × 20..60) and blue B (40..60 × 30..70) overlap in 40..60 × 30..60; the
+/// green sliver C (38..40 × 40..50), in front of both, meets that overlap at x = 40 over A only.
+#[test]
+fn raising_one_overlap_keeps_an_adjacent_third_member_above_its_seam() {
+    let mut s = session();
+    let rect = |s: &mut Session, x: f64, y: f64, w: f64, h: f64, color: &str| {
+        let id = id_of(&s.execute("shape.rectangle", &json!({"x": x, "y": y, "width": w, "height": h})).unwrap());
+        s.execute("paint.setFill", &json!({"ids": [id.0], "color": color})).unwrap();
+        s.execute("paint.setStroke", &json!({"ids": [id.0], "none": true})).unwrap();
+        id
+    };
+    let a = rect(&mut s, 20.0, 20.0, 60.0, 40.0, "#ff0000");
+    let b = rect(&mut s, 40.0, 30.0, 20.0, 40.0, "#0000ff");
+    let c = rect(&mut s, 38.0, 40.0, 2.0, 10.0, "#00ff00");
+    select(&mut s, &[a, b, c]);
+    let g = id_of(&s.execute("object.intertwine.make", &json!({})).unwrap());
+    let r = render(&s);
+    assert!(green(&r, 39, 45) && blue(&r, 50, 45) && red(&r, 39, 55));
+    let v = arrange(&mut s, json!({"at": [50, 45], "order": "front", "member": a.0}));
+    assert_eq!((&v["count"], &v["top"]), (&json!(1), &json!([a.0])));
+    assert_eq!(order_at(&s.doc().unwrap().doc, g, 50.0, 45.0), [b, a]);
+    let r = render(&s);
+    assert!(red(&r, 50, 45), "A is brought over B");
+    assert!(green(&r, 39, 45), "C stays in front of A within the margin");
+    assert!(red(&r, 39, 55));
+    let raised = area(&s, a).unwrap();
+    let inside = |x, y| vectorcraft_tools::intertwine::contains(&raised, vectorcraft_geom::Point::new(x, y));
+    assert!(inside(50.5, 45.5), "the overlap");
+    assert!(inside(39.5, 55.5), "the margin where only A paints");
+    assert!(!inside(39.5, 45.5), "the margin leaves out where C paints too");
+}
+
 #[test]
 fn clicking_the_same_overlap_again_swaps_back() {
     let mut s = session();