anchor.diff:            ;; needs to be computed initially
             (t
              (let* ((parent (sheet-parent sheet))
  +		  (mirrored-ancestor (sheet-mirrored-ancestor parent))
                     (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)))))
  +		   (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)
  
positive.code:                    (when old-native-transformation
                       (let ((MT (compose-transformations
                                  (compose-transformations
                                   (sheet-native-transformation (sheet-parent sheet))
                                   (sheet-transformation sheet))
                                  (invert-transformation old-native-transformation))))
                         (multiple-value-bind (fits-p MR) (choose MT)
  
negative.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 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)
  