Fix grid snapping for box shape gizmos

"Extruding" box face (i.e. moving it without holding Alt key) will now
always snap new box size exactly to the nearest snap step.

editor/plugins/gizmos/gizmo_3d_helper.cpp

@@ -77,36 +77,46 @@ String Gizmo3DHelper::box_get_handle_name(int p_id) const {
 }
 
 void Gizmo3DHelper::box_set_handle(const Vector3 p_segment[2], int p_id, Vector3 &r_box_size, Vector3 &r_box_position) {
-	Vector3 axis;
-	axis[p_id / 2] = 1.0;
-	Vector3 ra, rb;
+	int axis = p_id / 2;
 	int sign = p_id % 2 * -2 + 1;
+
 	Vector3 initial_size = initial_value;
+	float neg_end = initial_size[axis] * -0.5;
+	float pos_end = initial_size[axis] * 0.5;
 
-	Geometry3D::get_closest_points_between_segments(Vector3(), axis * 4096 * sign, p_segment[0], p_segment[1], ra, rb);
-	if (ra[p_id / 2] == 0) {
-		// Point before half of the shape. Needs to be calculated in opposite direction.
-		Geometry3D::get_closest_points_between_segments(Vector3(), axis * 4096 * -sign, p_segment[0], p_segment[1], ra, rb);
-	}
+	Vector3 axis_segment[2] = { Vector3(), Vector3() };
+	axis_segment[0][axis] = 4096.0;
+	axis_segment[1][axis] = -4096.0;
+	Vector3 ra, rb;
+	Geometry3D::get_closest_points_between_segments(axis_segment[0], axis_segment[1], p_segment[0], p_segment[1], ra, rb);
 
-	float d = ra[p_id / 2] * sign;
+	// Calculate new size.
+	r_box_size = initial_size;
+	if (Input::get_singleton()->is_key_pressed(Key::ALT)) {
+		r_box_size[axis] = ra[axis] * sign * 2;
+	} else {
+		r_box_size[axis] = sign > 0 ? ra[axis] - neg_end : pos_end - ra[axis];
+	}
 
-	Vector3 he = r_box_size;
-	he[p_id / 2] = d * 2;
+	// Snap to grid.
 	if (Node3DEditor::get_singleton()->is_snap_enabled()) {
-		he[p_id / 2] = Math::snapped(he[p_id / 2], Node3DEditor::get_singleton()->get_translate_snap());
+		r_box_size[axis] = Math::snapped(r_box_size[axis], Node3DEditor::get_singleton()->get_translate_snap());
 	}
+	r_box_size[axis] = MAX(r_box_size[axis], 0.001);
 
+	// Adjust position.
 	if (Input::get_singleton()->is_key_pressed(Key::ALT)) {
-		he[p_id / 2] = MAX(he[p_id / 2], 0.001);
-		r_box_size = he;
 		r_box_position = initial_transform.get_origin();
 	} else {
-		he[p_id / 2] = MAX(he[p_id / 2], -initial_size[p_id / 2] + 0.002);
-		r_box_size = (initial_size + (he - initial_size) * 0.5).abs();
-		Vector3 pos = initial_transform.affine_inverse().xform(initial_transform.get_origin());
-		pos += (r_box_size - initial_size) * 0.5 * sign;
-		r_box_position = initial_transform.xform(pos);
+		if (sign > 0) {
+			pos_end = neg_end + r_box_size[axis];
+		} else {
+			neg_end = pos_end - r_box_size[axis];
+		}
+
+		Vector3 offset;
+		offset[axis] = (pos_end + neg_end) * 0.5;
+		r_box_position = initial_transform.xform(offset);
 	}
 }