cur_val is the value that corresponds to the position of the scroll box
(also called the thumb). It returns a value between zero and max_val
page_step is the amount that cur_val is to be increased/decreased
when the user clicks on the scroll bar (other than on the thumb). For a
document, page_step will be set to the number of lines that are to be
displayed in the window and max_val should be set to n - page_step,
where n is the total number of lines in the document.
With %hx and %vx - Vertical Scrollbar the three arguments cur_val, page_step and max_val
can be passed as variables and changed dynamically with a call to
A call to clearwin_info@('INTERMEDIATE_SCROLL') will determine if the callback
was called whilst the thumb was moving. A return value of 1 indicates movement
whilst zero indicates that the thumb is stationary. Both of these events occur
at the end of a move.
%hx and %vx - Vertical Scrollbar should not be used with controls like
%eb - Edit Box and %cw - ClearWin Window which have their
own scroll bar mechanism.