Problem:    Double free when list is copied.
Solution:   Allocate the type when making a copy. (closes #8862)
            Clear the type for flattennew().  Avoid a memory leak when
            flattennew() fails.

Problem:    Crash when deleting a listener in a listener callback. (Naohiro
            Ono)
Solution:   Mark the listener and delete it later.

Problem:    Warning for using uninitialized variable.
Solution:   Initialize it. (John Marriott)

Problem:    A sequence of spaces is hard to see in list mode.
Solution:   Add the "multispace" option to 'listchars'. (closes #8834)

Problem:    Vim9: list += list creates a new list in :def function.
Solution:   Append to the existing list.

Problem:    Vim9: no failure if return type differs from returned variable.
Solution:   Copy type when copying a list. (closes #8847)

Problem:    A bit of code is not covered by tests.
Solution:   Add a few more test cases. (Dominique Pellé, closes #8857)

Problem:    _REENTRANT defined more than once.
Solution:   Fix configure script. (Christian Brabandt, closes #8852)

Problem:    A failing debug expression may make Vim unusable.
Solution:   Suppress error messages. (closes #8848)