Problem:    Insufficient testing for reading/writing files.
Solution:   Add more tests. (Yegappan Lakshmanan, closes vim/vim#6257)
            Add "ui_delay" to test_override() and use it for the CTRL-O test.
https://github.com/vim/vim/commit/b340baed9f7fc1c19a0742e2214d54982190c15e

Omit test_override().
Reorder test_writefile.vim to match Vim.

Problem:    Code for modeless selection not sufficiently tested.
Solution:   Add tests.  Move mouse code functionality to a common script file.
            (Yegappan Lakshmanan, closes vim/vim#5821)
https://github.com/vim/vim/commit/515545e11f523d14343b1e588dc0b9bd3d362bc2

Skip termcode tests for now.

Problem:    When an internal error makes Vim exit the error is not seen.
Solution:   Add the error to the test output.
https://github.com/vim/vim/commit/1c67f3a9779b99bed7aacb3108abbb649445d3ed

Add emsg_not_now() check to make code equivalent.

Let :wincmd command accept a count like what its documentation suggests.
Previously it could only accept a range, which led to some ambiguity on
which attribute should be used when executing :wincmd using nvim_cmd.

Closes #19662.

Also fix a typo in a related Vim test:

vim-patch:9.0.0223: typo in diffmode test

Problem:    Typo in diffmode test.
Solution:   Fix the typo. (closes vim/vim#10932)
https://github.com/vim/vim/commit/5fd6ab820b4a0aaa5c6020852f39d118375fab49

Problem:    Messages test fails; window size incorrect when 'cmdheight' is
            made smaller.
Solution:   Properly cleanup after test with cmdheight zero.  Resize windows
            correctly when 'cmdheight' gets smaller.
https://github.com/vim/vim/commit/d4cf9fc53e0b1d36e84d28ecd5595a6f102f325e

N/A patches for version.c:

vim-patch:9.0.0192: possible invalid memory access when 'cmdheight' is zero

Problem:    Possible invalid memory access when 'cmdheight' is zero. (Martin
            Tournoij)
Solution:   Avoid going over the end of w_lines[] when w_height is Rows.
            (closes vim/vim#10882)
https://github.com/vim/vim/commit/fdc5d17d58cc9c9edc9fb2816e1afaabc531bf1e

Problem:    The way 'cmdheight' can be made zero is inconsistent.
Solution:   Only make 'cmdheight' zero when setting it explicitly, not when
            resizing windows. (closes vim/vim#10890)
https://github.com/vim/vim/commit/f797e309caff48f7a56c73b16e62ff67c4dcbdd6

Problem:    No good reason why text objects are only in larger builds.
Solution:   Graduate +textobjects.
https://github.com/vim/vim/commit/887748742deae3d6de7aa0fdbb042afe1ccf5e7a

Problem:    Invalid memory access with for loop over NULL string.
Solution:   Make sure mb_ptr2len() consistently returns zero for NUL.
https://github.com/vim/vim/commit/f6d39c31d2177549a986d170e192d8351bd571e2

Problem:    The argument list may contain duplicates.
Solution:   Add the :argdedeupe command. (Nir Lichtman, closes vim/vim#6235)
https://github.com/vim/vim/commit/73a024209cbfbd5b39a2e974084d807c6131e2ed

Use latest index.txt :argdedupe doc from Vim.

Problem:    Undo earlier test sometimes fails on MS-Windows.
Solution:   Use another file name.
https://github.com/vim/vim/commit/cce293f87beb57a75ff738fade7fafadbc4a78a9