Problem: Display errors when making topline shorter and 'smoothscroll' is
set.
Solution: Reset w_skipcol when the topline becomes shorter than its current
value. (Luuk van Baal, closes#12367)
Problem: Cursor positioning and display problems with 'smoothscroll' and
using "zt", "zb" or "zz".
Solution: Adjust computations and conditions. (Yee Cheng Chin,
closes#11764)