Enables and initializes manual scrollbars for the canvas. A horizontal or vertical scrollbar can be activated only in a canvas that was created with the 'hscroll or 'vscroll style flag, respectively.
With manual scrollbars, the programmer is responsible for managing all details of the scrollbars, and the scrollbar state has no effect on the canvas's virtual size. Instead, the canvas's virtual size is the same as its client size.
See also init-auto-scrollbars for information about automatic scrollbars. The horizontal and vertical scrollbars are always either both manual or both automatic, but they are independently enabled. Automatic scrollbars can be re-initialized as manual, and vice-versa.
The h-length and v-length arguments specify the length of each scrollbar in scroll steps (i.e., the maximum value of each scrollbar). If either is #f, the scrollbar is disabled in the corresponding direction.
The h-page and v-page arguments set the number of scrollbar steps in a page, i.e., the amount moved when pressing above or below the value indicator in the scrollbar control.
The h-value and v-value arguments specify the initial values of the scrollbars.
If h-value is greater than h-length or v-value is greater than v-length, an exn:application:mismatch exception is raised. (The page step may be larger than the total size of a scrollbar.)