When the sheet-transformation of sheet is changed, the sheet-native-transformation is not updated for mirrored-sheet-mixins. As a result, the stream-pointer-position returns incorrect pointer position on scrolled sheet which is calculated using the sheet-native-transformation. I think this is because (setf native-transformation nil) is commented in (defmethod invalidate-cached-transformations ((sheet mirrored-sheet-mixin)) with strange description "XXX hm...". The method (defmethod sheet-native-transformation ((sheet mirrored-sheet-mixin)) does nothing if native-transformation has not been invalidated. These methods are defined in sheets.lisp
In attached example I create the pane 'canvas' with :min-height 2000 :min-width 2000 and layout (scrolling (:height 500 :width 500) ...). In define-presentation-action for blank-area I call stream-pointer-position and print string "TEST" at this point. In Allegro's CLIM implementation the strings appear where I have pressed pointer button; in McCLIM they appear only at the first page of canvas, even if I have pressed button on blank-area on the other pages.
