Position of this buffer's vertical scroll bar.
The value takes effect whenever you tell a window to display this buffer;
for instance, with `set-window-buffer' or when `display-buffer' displays it.
A value of `left' or `right' means put the vertical scroll bar at that side
of the window; a value of nil means don't show any vertical scroll bars.
A value of t (the default) means do whatever the window's frame specifies.