Y offset, in pixels, for the display of tooltips.
The offset is the distance between the Y position of the mouse and
the top border of the tooltip window. It must be chosen so that the
tooltip window doesn't contain the mouse when it pops up, or it may
interfere with clicking where you wish.
If `tooltip-frame-parameters' includes the `top' parameter,
the value of `tooltip-y-offset' is ignored.