Problem:    Tests using null list or dict fail.
Solution:   Only use the new rules for Vim9 script.

Problem:    Vim9: cannot initialize a variable to null_list.
Solution:   Give negative count to NEWLIST. (closes #10027)
            Also fix inconsistencies in comparing with null values.

Problem:    Visual range does not work before command modifiers.
Solution:   Move Visual range to after command modifiers.

Problem:    Using freed memory in flatten().
Solution:   Clear typval after recursing into list.

Problem:    Crash when switching window in BufWipeout autocommand.
Solution:   Put any buffer in the window to avoid it being NULL.
            (closes #10024)

Problem:    'cursorline' not always updated with 'cursorlineopt' is
            "screenline".
Solution:   Call check_redraw_cursorline() more often. (closes #10013)

Problem:    flattennew() makes a deep copy unnecessarily.
Solution:   Use a shallow copy. (issue #10012)

Problem:    Not enough testing for 2/3 letter substitute commands.
Solution:   Add more tests. (Yegappan Lakshmanan, closes #10019)

Problem:    flatten() does not use maxdepth correctly.
Solution:   Use a recursive implementation. (closes #10020)

Problem:    Visual area not fully updated when removing sign in Visual mode
            while scrolling.
Solution:   Adjust check for topline. (closes #10017)