fix(:bcd): do not "inherit" buffer-local dir

Problem:
Buffer-local CWD (:bcd) is "sticky", similar to window-local CWD (:lcd).
But this contradicts one of its main benefits: per-buffer "project root"
for LSP, OSC7.

Other problems:
- A buffer created with :edit/:enew/:new silently inherits b_localdir
  (and b_prevdir) from the previous buffer.
- curbuf_reusable() refuses to recycle a scratch buffer that has
  `b_localdir`.
- After :new/:vnew/:tabnew the CWD sticks to previous buffer's
  `b_localdir` even though the new curbuf has none, so :new is not
  equivalent to ":split | enew", and getcwd() disagrees with
  haslocaldir().
- Requires "which buffer spawned this buffer" semantics that no other
  buffer-local state has.

Solution:
Drop sticky/inherit behavior of buffer-local CWD (:bcd).

- do_ecmd: always apply the new curbuf's dir (`fix_current_dir`), like
  `do_autochdir` already does. :tabnew from a :bcd buffer now reverts to
  global CWD (and fires DirChanged), same as :tabnew from a :lcd window.
- curbuf_reusable(): recycling a scratch buffer frees its b_localdir.

To get sticky/inherit behavior of CWD, use `:lcd`.

src/nvim/buffer.c

@@ -1778,7 +1778,7 @@ void set_curbuf(buf_T *buf, int action, bool update_jumplist)
   }
 
   // Maybe cd to buffer-local directory
-  fix_current_dir(false);
+  update_cwd(kCdCauseBuffer);
 }
 
 /// Enter a new current buffer.
@@ -2031,8 +2031,6 @@ buf_T *buflist_new(char *ffname_arg, char *sfname_arg, linenr_T lnum, int flags)
     trigger_undo_ftplugin(buf, curwin);
     // It's like this buffer is deleted.  Watch out for autocommands that
     // change curbuf!  If that happens, allocate a new buffer anyway.
-    // We also ask it to not free the buffer-local directory so we can reuse
-    // it.
     buf_freeall(buf, BFA_WIPE | BFA_DEL);
     if (aborting()) {           // autocmds may abort script processing
       xfree(ffname);
@@ -2162,8 +2160,6 @@ bool curbuf_reusable(void)
           && curbuf->b_ffname == NULL
           && curbuf->b_nwindows <= 1
           && !curbuf->terminal
-          && curbuf->b_localdir == NULL
-          && curbuf->b_prevdir == NULL
           && (curbuf->b_ml.ml_mfp == NULL || buf_is_empty(curbuf))
           && !bt_quickfix(curbuf)
           && !curbufIsChanged());

src/nvim/context.c

@@ -583,7 +583,7 @@ void ctx_restore(CtxSwitch *cs)
     // If :lcd has been used in the autocommand window, correct current
     // directory before restoring b_localdir, tp_localdir and globaldir.
     if (cwp->w_localdir != NULL) {
-      fix_current_dir(true);
+      update_cwd(kCdCauseWindow);
     }
     if (bufref_valid(&cs->cs_new_curbuf)) {
       xfree(cs->cs_new_curbuf.br_buf->b_localdir);

src/nvim/eval.lua

@@ -1265,8 +1265,8 @@ M.funcs = {
 
       If {scope} is present, changes the current working directory
       for the specified scope:
-          "window"	Changes the window local directory.  |:lcd|
           "buffer"	Changes the buffer local directory.  |:bcd|
+          "window"	Changes the window local directory.  |:lcd|
           "tabpage"	Changes the tabpage local directory.  |:tcd|
           "global"	Changes the global directory.  |:cd|
 
@@ -4101,23 +4101,27 @@ M.funcs = {
       is ignored.
 
       Tabs, windows and buffers are identified by their respective
-      numbers, 0 means current tab or window or buffer. Missing tab
-      number
-      implies 0. Thus the following are equivalent: >vim
+      numbers, 0 means current tab/window/buffer. Missing {tabnr}
+      implies 0 (missing {bufnr} does not; see below). Thus the
+      following are equivalent: >vim
       	getcwd(0)
       	getcwd(0, 0)
       <If {winnr} is -1 it is ignored, only the tab is resolved.
       {winnr} is a |window-number| or |window-ID|.
+
       If both {winnr} and {tabnr} are -1 and {bufnr} is missing the
       global working directory is returned.
+
       Note: When {tabnr} is -1 Vim returns an empty string to
       signal that it is invalid, whereas Nvim returns either the
       global working directory if {winnr} is -1 or the working
       directory of the window indicated by {winnr}.
 
-      If {bufnr} is provided, {winnr} and {tabnr} must be -1 and the
-      working directory of that buffer is returned. An argument may
-      be -1 only if all preceding arguments are -1.
+      If {bufnr} is provided, {winnr} and {tabnr} must be -1, then
+      the buffer-local working directory is returned.
+
+      An argument may be -1 only if all preceding arguments are -1.
+
       Examples of buffer usage: >vim
             getcwd(-1, -1, 0)  " Get current buffer's directory
             getcwd(-1, -1, 3)  " Get directory of buffer #3
@@ -5435,8 +5439,9 @@ M.funcs = {
       and {bufnr} has set a local path via |:bcd|, otherwise 0.
 
       Tabs, windows and buffers are identified by their respective
-      numbers, 0 means current tab, window or buffer. Missing
-      argument implies 0. Thus the following are equivalent: >vim
+      numbers, 0 means current tab/window/buffer. Missing {winnr}
+      or {tabnr} implies 0 (missing {bufnr} does not; see below).
+      Thus the following are equivalent: >vim
       	echo haslocaldir()
       	echo haslocaldir(0)
       	echo haslocaldir(0, 0)

src/nvim/ex_cmds.c

@@ -2937,23 +2937,8 @@ int do_ecmd(int fnum, char *ffname, char *sfname, exarg_T *eap, linenr_T newlnum
     redraw_curbuf_later(UPD_NOT_VALID);  // redraw this buffer later
   }
 
-  // A new buffer inherits the buffer-local directory of the buffer it was created from (unless
-  // autocommands already set one).
-  if (!oldbuf && curbuf->b_localdir == NULL
-      && bufref_valid(&old_curbuf)
-      && old_curbuf.br_buf != curbuf
-      && old_curbuf.br_buf->b_localdir != NULL) {
-    curbuf->b_localdir = xstrdup(old_curbuf.br_buf->b_localdir);
-    if (old_curbuf.br_buf->b_prevdir != NULL) {
-      curbuf->b_prevdir = xstrdup(old_curbuf.br_buf->b_prevdir);
-    }
-  }
-
-  // If editing a buffer in the current window, make sure to update to the
-  // buffer's working directory.
-  if (oldwin == curwin) {
-    fix_current_dir(false);
-  }
+  // Update to the working directory of the new current buffer.
+  update_cwd(kCdCauseBuffer);
 
   // Change directories when the 'acd' option is set.
   do_autochdir();

src/nvim/options.lua

@@ -1093,7 +1093,7 @@ local options = {
         doc = [[on on Unix, off on Windows]],
       },
       desc = [=[
-        When on, |:cd|, |:tcd| and |:lcd| without an argument changes the
+        When on, |:cd|, |:tcd|, |:lcd| and |:bcd| without an argument changes the
         current working directory to the |$HOME| directory like in Unix.
         When off, those commands just print the current directory name.
       ]=],
@@ -1113,7 +1113,7 @@ local options = {
       deny_duplicates = true,
       desc = [=[
         This is a list of directories which will be searched when using the
-        |:cd|, |:tcd| and |:lcd| commands, provided that the directory being
+        |:cd|, |:tcd|, |:lcd| and |:bcd| commands, provided that the directory being
         searched for has a relative path, not an absolute part starting with
         "/", "./" or "../", the 'cdpath' option is not used then.
         The 'cdpath' option's value has the same form and semantics as
@@ -2119,7 +2119,7 @@ local options = {
         		following occurrence.
         							*cpo-~*
         	~	When included, don't resolve symbolic links when
-        		changing directory with |:cd|, |:lcd|, or |:tcd|.
+        		changing directory with |:cd|, |:tcd|, |:lcd|, or |:bcd|.
         		This preserves the symbolic link path in buffer names
         		and when displaying the current directory.  When
         		excluded (default), symbolic links are resolved to

src/nvim/window.c

@@ -5300,7 +5300,7 @@ static void win_enter_ext(win_T *const wp, const int flags)
     win_fix_cursor(get_real_state() & (MODE_NORMAL|MODE_CMDLINE|MODE_TERMINAL));
   }
 
-  fix_current_dir(true);
+  update_cwd(kCdCauseWindow);
 
   entering_window(curwin);
   // Careful: autocommands may close the window and make "wp" invalid
@@ -5352,11 +5352,12 @@ static void win_enter_ext(win_T *const wp, const int flags)
   do_autochdir();
 }
 
-/// Used after making another window or buffer the current one: change directory if needed.
-void fix_current_dir(bool caused_by_win)
+/// Applies the effective current-directory of the current window (its window-local, buffer-local,
+/// tab-local or the global directory). Called after setting curbuf/curwin.
+///
+/// @param cause What caused the switch, reported by the DirChanged event.
+void update_cwd(CdCause cause)
 {
-  CdCause cause = caused_by_win ? kCdCauseWindow : kCdCauseBuffer;
-
   // New directory is either the local directory of the window, buffer, tab or NULL.
   char *new_dir;
   CdScope scope;