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