Fix #322: dragged direction handles take Shift and smart guides

Dragging a direction handle with Direct Selection or the Anchor Point tool
sent the pointer position straight to path.setHandle: Shift did nothing and
the handle never snapped, while the Pen already constrains the handles it
pulls out.

guides::snap_handle places a dragged handle: with Shift at a multiple of
45° from its anchor (the Pen's constraint, keeping the dragged length),
otherwise snapped as a drawn point is (smart guides, the grid, pixels).
Both tools use it and show the smart guides while the handle moves.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

crates/engine/src/tests_draw2.rs

@@ -389,3 +389,63 @@ fn draw2_commands_reject_bad_params() {
         assert_eq!(s.doc().unwrap().history.undo.len(), before, "{cmd} left an undo step");
     }
 }
+
+/// A curve from (100, 100) to (300, 100) whose first anchor has an out handle at (150, 100).
+fn curve(s: &mut Session) -> NodeId {
+    let anchors = json!([{"x": 100, "y": 100, "out": [150, 100]}, {"x": 300, "y": 100, "in": [250, 100]}]);
+    NodeId(s.execute("path.create", &json!({"anchors": anchors})).unwrap()["id"].as_u64().unwrap())
+}
+
+/// Drag the out handle of path `id`'s first anchor with `tool` to `to`, holding `mods`. Direct
+/// Selection clicks the anchor first, so its handles show.
+fn drag_handle(s: &mut Session, tool: &str, id: NodeId, to: (f64, f64), mods: Mods, v: ViewInfo) {
+    let from = path(s, id).subpaths[0].anchors[0].h_out;
+    s.execute("select.set", &json!({"ids": [id.0]})).unwrap();
+    s.select_tool(tool, v).unwrap();
+    if tool == "directSelection" {
+        s.execute("select.none", &json!({})).unwrap();
+        s.pointer(&PointerEvent::new(PointerKind::Down, 100.0, 100.0), v).unwrap();
+        s.pointer(&PointerEvent::new(PointerKind::Up, 100.0, 100.0), v).unwrap();
+    }
+    s.pointer(&PointerEvent::new(PointerKind::Down, from.x, from.y).with_mods(mods), v).unwrap();
+    s.pointer(&PointerEvent::new(PointerKind::Drag, to.0, to.1).with_mods(mods), v).unwrap();
+    s.pointer(&PointerEvent::new(PointerKind::Up, to.0, to.1).with_mods(mods), v).unwrap();
+    assert!(!s.in_interaction());
+}
+
+/// The out handle of path `id`'s first anchor.
+fn out_handle(s: &Session, id: NodeId) -> Point {
+    path(s, id).subpaths[0].anchors[0].h_out
+}
+
+#[test]
+fn shift_keeps_a_dragged_handle_at_45_degree_steps() {
+    // #322.
+    let shift = Mods { shift: true, ..Mods::default() };
+    for tool in ["directSelection", "anchorPoint"] {
+        let mut s = session();
+        let id = curve(&mut s);
+        drag_handle(&mut s, tool, id, (190.0, 130.0), shift, view());
+        let h = out_handle(&s, id);
+        assert!(near(h, Point::new(100.0 + 90.0f64.hypot(30.0), 100.0)), "{tool}: {h:?}");
+        drag_handle(&mut s, tool, id, (160.0, 155.0), shift, view());
+        let h = out_handle(&s, id);
+        assert!((h.x - h.y).abs() < 1e-6 && h.x > 100.0, "{tool}: 45°: {h:?}");
+        // Without Shift the handle goes where it is dragged.
+        drag_handle(&mut s, tool, id, (190.0, 130.0), Mods::default(), view());
+        assert_eq!(out_handle(&s, id), Point::new(190.0, 130.0), "{tool}");
+    }
+}
+
+#[test]
+fn a_dragged_handle_snaps_to_smart_guides() {
+    // #322: in line with the other anchor, with smart guides on.
+    for tool in ["directSelection", "anchorPoint"] {
+        let mut s = session();
+        let id = curve(&mut s);
+        drag_handle(&mut s, tool, id, (298.0, 160.0), Mods::default(), ViewInfo::default());
+        assert_eq!(out_handle(&s, id), Point::new(300.0, 160.0), "{tool}");
+        drag_handle(&mut s, tool, id, (298.0, 170.0), Mods::default(), view());
+        assert_eq!(out_handle(&s, id), Point::new(298.0, 170.0), "{tool}: smart guides off");
+    }
+}