Problem: When `nvim_set_keymap` tries to overwrite a `<unique>` mapping,
it throws an error even when called in `pcall`.

Solution: src/nvim/mapping.c:buf_do_map no longer calls `semsg`. Its
callers now decide whether to ignore the error, or use
`semsg` (not caught)/`api_set_error` (caught by `pcall`).

When setting a keymap with "unique" that already exists the error
message contains the LHS of the keymap with termcodes replaced. In
particular this means that keys like <Tab> show as an actual tab
character, meaning the error message displays as "Mapping already exists
for ", which is hard to debug for users.

Instead, display the original LHS (without any simplification or parsed
termcodes). This rperesents exactly what the user passed to the `lhs`
argument of `nvim_set_keymap`, which makes it easier to find where the
offending keymap is.

Problem
Pasting a big string ("streamed paste" with multiple chunks) sets the '[
mark to the edit from the last chunk, instead of the start of the paste.

Solution:
Set the '[ mark where the paste started, not where the last chunk was
inserted.

Note: `startpos == nil` is not equal to `phase == 1` because there may
be some empty chunks pasted which won't arrive here (returned at code
before).

Problem: Reconfiguring a float window applies the global 'winborder'.
Solution:
- Ignore 'winborder' when reconfiguring a float window.
- Still apply 'winborder' when converting a split to a float window.

While at it, rename the p_winbd variable to p_winborder, as 'winbd' is
not the option's short name.

Co-authored-by: glepnir <glephunter@gmail.com>

Problem:    Sourcing buffer lines is too complicated.
Solution:   Simplify the code. Make it possible to source Vim9 script lines.
            (Yegappan Lakshmanan, closes vim/vim#9974)

https://github.com/vim/vim/commit/85b43c6cb7d56919e245622f4e42db6d8bee4194

This commit changes the behavior of sourcing buffer lines to always have
a script ID, although sourcing the same buffer always produces the same
script ID.

vim-patch:9.1.0372: Calling CLEAR_FIELD() on the same struct twice

Problem:  Calling CLEAR_FIELD() on the same struct twice.
Solution: Remove the second CLEAR_FIELD().  Move the assignment of
          cookie.sourceing_lnum (zeertzjq).

closes: vim/vim#14627

https://github.com/vim/vim/commit/f68517c1671dfedcc1555da50bc0b3de6d2842f6

Co-authored-by: Yegappan Lakshmanan <yegappan@yahoo.com>

Implement an extmark property that conceals lines vertically.

Add a new field `virt_lines_overflow` that enables horizontal scrolling
for virtual lines when set to "scroll".