anchor.diff:  (defclass generic-tree-pane (tree-pane meta-tree-pane
                                          standard-sheet-input-mixin ;; Hmm..
                                          value-changed-repaint-mixin
  -                                       mouse-wheel-scroll-mixin)
  +                                       mouse-wheel-scroll-mixin
  +                                       sheet-padding-mixin)
     ((highlight-ink :initform +royalblue4+
                     :initarg :highlight-ink
                     :reader tree-pane-highlight-ink)
  
positive.code:                  :documentation "Last action performed on items in the pane, either
  :select, :deselect, or NIL if none has been performed yet."))
    (:default-initargs :text-style (make-text-style :sans-serif :roman :normal)
      :background +white+ :foreground +black+))
  
  (defmethod initialize-instance :after ((gadget meta-tree-pane) &rest rest)
    (declare (ignorable rest))
  
negative.code: 
            ;; Draw the node's children y the node is opened
  
            (let ((next-region (make-bounding-rectangle x0
                                                        (+ y0 item-height)
                                                        x1
                                                        y1)))
              (when (member value (opened-nodes pane) :test (tree-pane-test pane))
                (loop
                   with children-indentation = (+ current-indentation indentation)
  