Problem:    BS and DEL do not work properly in an interacive shell. (Gary
            Johnson)
Solution:   Adjust the length for replaced codes.

Problem:    'smoothscroll' not tested with 'number' and "n" in 'cpo'.
Solution:   Add tests, fix uncovered problem.

Problem:    Build fails without the +conceal feature.
Solution:   Rename called function.

Problem:    Some tests are failing.
Solution:   Adjust for "<<<" showing up.

Problem:    No indication when the first line is broken for 'smoothscroll'.
Solution:   Show "<<<" in the first line.

Problem:    When using powershell input redirection does not work.
Solution:   Use a different shell command for powershell. (Yegappan
            Lakshmanan, closes #11257)

Problem:    The 'splitscroll' option is not a good name.
Solution:   Rename 'splitscroll' to 'splitkeep' and make it a string option,
            also supporting "topline". (Luuk van Baal, closes #11258)

Problem:    with 'smoothscroll' set CTRL-E does not work properly when
            'foldmethod' is set to "indent". (Yee Cheng Chin)
Solution:   Merge the code for scroling with folds and 'smoothscroll'.
            (closes #11262)

Problem:    CTRL-Y does not stop at line 1. (John Marriott)
Solution:   Stop at line 1 when 'smoothscroll' is not set. (closes #11261)