Smart Mouse Drags within a Window

If a region is active and a drag occurs within a single buffer/window:
  ACTION KEY
     Restores region to before Action Key drag and signals an error.
  ASSIST KEY
     Restores region to before Action Key drag and signals an error.

(Note that hmouse-x-drag-sensitivity sets the minimal horizontal
movement which registers a drag).  If dragged horizontally within a
single window from anywhere but a thing delimiter:
  ACTION KEY
     Splits the current window, adding a window below.
  ASSIST KEY
     Deletes the current window if it is not the sole window in the
     current frame.

(Note that hmouse-y-drag-sensitivity sets the minimal vertical
movement which registers a drag).  If dragged vertically within a
single window from anywhere but a thing delimiter:
  ACTION KEY
     Splits the current window, adding a window to the right.
  ASSIST KEY
     Deletes the current window if it is not the sole window in the
     current frame.

If dragged diagonally within a single window while depressed
(‘hmouse-x-diagonal-sensitivity’ and ‘hmouse-y-diagonal-sensitivity’ set
the minimal diagonal movements which register a drag):
  ACTION KEY
     Saves the window configuration for the selected frame onto a ring
     of window configurations.
  ASSIST KEY
     Restores the prior window configuration from the ring.  A prefix
     argument N specifies the Nth prior configuration from the ring.