Problem:    With 'smoothscroll' skipcol may be reset unnecessarily.
Solution:   Check the line does actually fit in the window.

Problem:    "gk" may reset skipcol when not needed.
Solution:   Only reset skipcol if the cursor column is less.

Problem:    With 'smoothscroll' set mouse click position may be wrong.
Solution:   Adjust computations for w_skipcol. (Yee Cheng Chin, closes #11514)

Problem:    With 'smoothscroll' cursor may end up in wrong position.
Solution:   Correct the computation of screen lines. (Yee Cheng Chin,
            closes #11502)

Problem:    Cursor moves too far with 'smoothscroll'.
Solution:   Only move as far as really needed. (Yee Cheng Chin, closes #11504)

Problem:    With 'smoothscroll' cursor is one screen line too far down. (Ernie
            Rael)
Solution:   Add a test that currently has the wrong result so that a fix can
            be made. (issue #11436)

Problem:    'smoothscroll' cursor calculations wrong when 'number' is set.
Solution:   Correct the code that computes the width. (closes #11492)

Problem:    "precedes" from 'listchars' overwritten by <<< for 'smoothscroll'.
Solution:   Keep the "precedes" character.

Problem:    Line number not visisble with 'smoothscroll', 'nu' and 'rnu'.
Solution:   Put the ">>>" after the line number instead of on top.

Problem:    Cursor position invalid when scrolling with 'smoothscroll' set.
            (Ernie Rael)
Solution:   Add w_valid_skipcol and clear flags when it changes.  Adjust
            w_skipcol after moving the cursor.