Size of the vertical scrollbar's thumbnail in any unit.
Any floating-point value greater than zero and smaller than the difference between YMAX and YMIN.
LINEY, YMAX and YMIN are only updated in the scrollbar when DY is updated.
When the canvas is visible, a change in DY can generate a redraw in the horizontal scrollbar on the screen. But it may generate a RESIZE_CB callback event if YAUTOHIDE=Yes.
A change in these values can affect the attribute POSY.