Self half screen scrolling of "visual-line"

[jk411]

Dashes are edges of window,
[C] is cursor placement - it can be anywhere on the very bottom line of window,
last line has to be wrapping with only part of it visible

setup:

+--------
text \n
text \n
more text \n
text line \n
\n
very long [C] line that is wrapped and
+-------
part of it is not visible... \n

• starting "visual-line" mode causes window to scroll half-screen - wrapped line is window-centered