At the moment, the Gantt has no such functionality.
As a workaround, you can overwrite the internal _resizeScrollbars method in order to prevent hiding not active scrollbars.
Assign it an empty function during the onGanttLayoutReady event.
After that, the scrollbars will always display.
Here is a sample, where you can see how to do it:
I can not guarantee that this method will work after updates of the gantt in the future.