anchor.diff:                     (when old-native-transformation
                        (let ((MT (compose-transformations
                                   (compose-transformations
  -                                 (sheet-native-transformation (sheet-parent sheet))
  +                                 (sheet-native-transformation parent)
                                    (sheet-transformation sheet))
                                   (invert-transformation old-native-transformation))))
                          (multiple-value-bind (fits-p MR) (choose MT)
  
positive.code:              (with-bounding-rectangle* (mx1 my1 mx2 my2)
  				       sheet-region-in-native-parent
                 (let (;; pw, ph is the width/height of the parent
                       (pw  (bounding-rectangle-width (sheet-mirror-region parent)))
                       (ph  (bounding-rectangle-height (sheet-mirror-region parent))))
                   (labels ((choose (MT)
                              ;; -> fits-p mirror-region
                              (multiple-value-bind (x1 y1) (transform-position MT 0 0)
  
negative.code:                                   (compose-transformations
                                     (invert-transformation MT)
                                     (compose-transformations
                                      (sheet-native-transformation parent)
                                      (sheet-transformation sheet)))))
                               ;; finally reflect the change to the host window system
                               (setf (%sheet-mirror-region sheet) MR)
  