anchor.diff:               (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))))
  +                     (pw  (bounding-rectangle-width (sheet-mirror-region mirrored-ancestor)))
  +                     (ph  (bounding-rectangle-height (sheet-mirror-region mirrored-ancestor))))
                    (labels ((choose (MT)
                               ;; -> fits-p mirror-region
                               (multiple-value-bind (x1 y1) (transform-position MT 0 0)
  
positive.code:           ;; needs to be computed initially
            (t
             (let* ((parent (sheet-parent sheet))
                    (sheet-region-in-native-parent
                     ;; this now is the wanted sheet mirror region
                     (transform-region (sheet-native-transformation parent)
                                       (transform-region (sheet-transformation sheet)
                                                         (sheet-region sheet)))))
               (when (region-equal sheet-region-in-native-parent +nowhere+)
                 ;; hmm
                 (setf (%sheet-mirror-transformation sheet)
  
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)
  