WM_MBUTTONDBLCLK

  These messages are sent to a window when the middle mouse button is
pressed, released, and double- clicked  respectively. If a window has
captured the mouse  by calling SetCapture(), then this message will be sent
to that window  regardless of where the mouse is on the screen.

Parameters
  wParam is set to the shift state of the keyboard. This lets the
   application distinguish, for example, between moving the mouse with no
   keys held down and moving the mouse with the shift key held down.
  LOWORD(lParam) is the 0-based screen coordinate of the column that the
   mouse is on.
  HIWORD(lParam) is the 0-based screen coordinate of the row that the mouse
   is on.