anchor.diff:                 (MT (make-translation-transformation x1 y1))
                  (MR (make-rectangle* 0 0 (round (- x2 x1)) (round (- y2 y1))))
                  (native-transformation
  -                ;; NT = T o PNT o -MT
  -                (compose-transformations
  -                 (invert-transformation MT)
  -                 (compose-transformations (sheet-native-transformation (sheet-parent sheet))
  -                                          (sheet-transformation sheet))))
  +                 ;; NT = T o PNT o MT⁻¹
  +                 (compose-transformations
  +                  (invert-transformation MT)
  +                  (compose-transformations (sheet-native-transformation (sheet-parent sheet))
  +                                           (sheet-transformation sheet))))
                  (old-native-transformation
  -                (%%sheet-native-transformation sheet)))
  +                 (%%sheet-native-transformation sheet)))
   
             (cond ((and (> (round (- x2 x1)) 0)
                         (> (round (- y2 y1)) 0))
  
positive.code:   ;; The native transformation has to changed or needs to be computed initially.
    (let* ((parent (sheet-parent sheet))
           (sheet-region-in-native-parent
            ;; this now is the wanted sheet mirror region
            (transform-region (sheet-native-transformation parent)
                              (region-intersection (sheet-region parent)
                                                   (transform-region (sheet-transformation sheet)
                                                                     (sheet-region sheet))))))
      (when (region-equal sheet-region-in-native-parent +nowhere+)
        (%set-mirror-geometry sheet :invalidate-transformations t)
        (return-from update-mirror-geometry))
      ;; mx1 .. my2 are is now the wanted mirror region in the
      ;; parent coordinate system.
      (with-bounding-rectangle* (mx1 my1 mx2 my2)
          sheet-region-in-native-parent
        (let (;; pw, ph is the width/height of the mirror containing our sheet
              (pw (bounding-rectangle-width (%sheet-mirror-region* (sheet-mirrored-ancestor parent))))
              (ph (bounding-rectangle-height (%sheet-mirror-region* (sheet-mirrored-ancestor parent)))))
          (labels ((choose (MT)
                     ;; -> fits-p mirror-region
                     (multiple-value-bind (x1 y1) (transform-position MT 0 0)
  
negative.code:   - the parent's transformation changed
    - the parent's mirror region changed
  
  Also if the sheet's native transformation changes, the mirror's contents need
  to be redrawn, which is achieved by calling PORT-DIRTY-MIRROR-REGION. TODO is this true?
  
  Since changing the sheet's native transformation might thus be expensive,
  this function tries to minimize changes to it. (although it does not try
  