This change fixes the barnowl scrolling so that the ratios of
1/5, 1/2, and 2/3 are all relative to the visible viewport.
The core part of the fix is that we need to call
ui.message_viewport_info() to get the correct offsets for
the top/height of the actually visible viewport Because
this function is fairly expensive, I didn't want the
helper functions needlessly re-calling it. I could
have passed the helper functions the structure, but at
that point it was simpler to inline their functionality.
(imported from commit fb10d99313d568e85acfa70703c4651466acbc21)
66b93d5cfd