refactor(change): do API changes to buffer without curbuf switch

Most of the messy things when changing a non-current buffer is
not about the buffer, it is about windows. In particular, it is about
`curwin`.

When editing a non-current buffer which is displayed in some other
window in the current tabpage, one such window will be "borrowed" as the
curwin. But this means if two or more non-current windows displayed the buffers,
one of them will be treated differenty. this is not desirable.

In particular, with nvim_buf_set_text, cursor _column_ position was only
corrected for one single window. Two new tests are added: the test
with just one non-current window passes, but the one with two didn't.

Two corresponding such tests were also added for nvim_buf_set_lines.
This already worked correctly on master, but make sure this is
well-tested for future refactors.

Also, nvim_create_buf no longer invokes autocmds just because you happened
to use `scratch=true`. No option value was changed, therefore OptionSet
must not be fired.

1 files changed, 13 insertions, 0 deletions

diff --git a/src/nvim/optionstr.c b/src/nvim/optionstr.c
index f07c05c113..07e2e5eed1 100644
--- a/src/nvim/optionstr.c
+++ b/src/nvim/optionstr.c
@@ -406,6 +406,19 @@ void set_string_option_direct_in_win(win_T *wp, const char *name, int opt_idx, c
   unblock_autocmds();
 }
 
+/// Like set_string_option_direct(), but for a buffer-local option in "buf".
+/// Blocks autocommands to avoid the old curwin becoming invalid.
+void set_string_option_direct_in_buf(buf_T *buf, const char *name, int opt_idx, const char *val,
+                                     int opt_flags, int set_sid)
+{
+  buf_T *save_curbuf = curbuf;
+
+  block_autocmds();
+  curbuf = buf;
+  set_string_option_direct(name, opt_idx, val, opt_flags, set_sid);
+  curbuf = save_curbuf;
+  unblock_autocmds();
+}
 /// Set a string option to a new value, handling the effects
 ///
 /// @param[in]  opt_idx  Option to set.