$ strer = "A huge string."
frame:
background Solid("#FFFFFF")
xysize (600, 620)
xalign 0.5
has viewport:
xysize (580, 620)
scrollbars "vertical"
draggable True
mousewheel True
text "[strer]" size 14 color "#000000"
Try something like this, apparently it needs a container to align the bar properly, best best is prolly a side like you've tried but it should contain a viewport, not be a part of it.
Still, since we have a background, we might as well use a frame. This works on a fresh project, you'll have to replace background with the pergament image. If it doesn't size itself properly, use:
frame:
background Frame("path_to_image", 10, 10)