Fix #2678: Ungroup no longer multiplies the group's opacity into its members

Ungroup multiplied each member's opacity by the group's, so a member of a
50% group stayed at 50% after the group was gone, and members that
overlapped showed through each other. Each member now keeps its own
opacity, and the group's opacity is removed with the group.

crates/engine/src/cmd/object/arrange.rs

@@ -129,7 +129,7 @@ pub(super) fn ungroup(s: &mut Session, _: &Value) -> Result<Value> {
                 {
                     *clipping = false;
                 }
-                c.opacity *= n.opacity;
+                // Each member keeps its own opacity; the group's opacity goes with the group.
                 new_sel.push(c.id);
                 out.push(c);
             }

crates/engine/src/tests/containers.rs

@@ -69,6 +69,28 @@ fn a_group_fill_paints_both_members_and_the_contents_slot_moves() {
     assert!(s.execute("appearance.moveItem", &json!({"from": "contents", "to": 1})).is_err());
 }
 
+#[test]
+fn ungroup_leaves_the_groups_opacity_with_the_group() {
+    let mut s = session();
+    let red = id_of(&run(&mut s, "shape.rectangle", json!({"x": 20, "y": 20, "width": 60, "height": 40})));
+    run(&mut s, "paint.setFill", json!({"color": "#ff0000"}));
+    let blue = id_of(&run(&mut s, "shape.rectangle", json!({"x": 100, "y": 20, "width": 60, "height": 40})));
+    run(&mut s, "paint.setFill", json!({"color": "#0000ff"}));
+    run(&mut s, "object.setProps", json!({"ids": [blue.0], "opacity": 40}));
+    run(&mut s, "select.set", json!({"ids": [red.0, blue.0]}));
+    let g = id_of(&run(&mut s, "object.group", json!({})));
+    run(&mut s, "object.setProps", json!({"ids": [g.0], "opacity": 50}));
+    run(&mut s, "object.ungroup", json!({}));
+    // Each member keeps its own opacity: the red square is opaque again and the blue one stays at 40%.
+    assert!((node(&s, red).opacity - 1.0).abs() < 1e-6, "{}", node(&s, red).opacity);
+    assert!((node(&s, blue).opacity - 0.4).abs() < 1e-6, "{}", node(&s, blue).opacity);
+    assert_eq!(pixel(&s, 40, 40), RED);
+    // One undo step brings the group back at 50%.
+    run(&mut s, "edit.undo", json!({}));
+    assert!((node(&s, g).opacity - 0.5).abs() < 1e-6);
+    assert!((node(&s, red).opacity - 1.0).abs() < 1e-6);
+}
+
 #[test]
 fn appearance_commands_target_the_object_or_its_contents() {
     let mut s = session();