patch 9.1.0170: Re-allow curwin == prevwin, but document it instead

Problem:  more places exist where curwin == prevwin, and it may even be
          expected in some cases.
Solution: revert v9.1.0001, but document that it's possible instead.
          (Sean Dewar)

I've had a change of heart for the following reasons:

- A quick 'n dirty [GitHub code
  search](https://github.com/search?q=%2F%28winnr%5C%28%5C%29%5Cs*%3D%3D%5Cs*winnr%5C%28%5B%27%22%5D%23%5B%27%22%5D%5C%29%7Cwinnr%5C%28%5B%27%22%5D%23%5B%27%22%5D%5C%29%5Cs*%3D%3D%5Cs*winnr%5C%28%5C%29%29%2F&type=code)
  reveals some cases where it's expected in the wild.

  Particularly, it made me aware `winnr() == winnr('#')` is possible when curwin
  is changed temporarily during the evaluation of a &statusline expression item
  (`%{...}`), and is used to show something different on the statusline
  belonging to the previous window; that behaviour wasn't changed in v9.1.0001,
  but it means curwin == prevwin makes sense in some cases.

- The definition and call sites of back_to_prevwin imply some expectation that
  prevwin == wp (== curwin) is possible, as it's used to skip entering the
  prevwin in that case.

- Prior to v9.1.0001, `:wincmd p` would not beep in the case that was patched in
  v9.1.0001, but now does. That resulted in #14047 being opened, as it affected
  the CtrlP plugin.

  I find it odd that `:wincmd p` had cases where it wouldn't beep despite doing
  nothing, but it may be preferable to keep things that way (or instead also
  beep if curwin == prevwin, if that's preferred).

- After more digging, I found cases in win_free_mem, enter_tabpage,
  aucmd_restbuf and qf_open_new_cwindow where curwin == prevwin is possible
  (many of them from autocommands). Others probably exist too, especially in
  places where curwin is changed temporarily.

fixes: #14047
closes: #14186

Signed-off-by: Sean Dewar <6256228+seandewar@users.noreply.github.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>

runtime/doc/builtin.txt

@@ -1,4 +1,4 @@
-*builtin.txt*	For Vim version 9.1.  Last change: 2024 Mar 07
+*builtin.txt*	For Vim version 9.1.  Last change: 2024 Mar 12
 
 
 		  VIM REFERENCE MANUAL	  by Bram Moolenaar
@@ -10947,7 +10947,9 @@ winnr([{arg}])	The result is a Number, which is the number of the current
 			#	the number of the last accessed window (where
 				|CTRL-W_p| goes to).  If there is no previous
 				window or it is in another tab page 0 is
-				returned.
+				returned.  May refer to the current window in
+				some cases (e.g. when evaluating 'statusline'
+				expressions).
 			{N}j	the number of the Nth window below the
 				current window (where |CTRL-W_j| goes to).
 			{N}k	the number of the Nth window above the current

src/globals.h

@@ -976,7 +976,7 @@ EXTERN int	clip_unnamed_saved INIT(= 0);
  */
 EXTERN win_T	*firstwin;		// first window
 EXTERN win_T	*lastwin;		// last window
-EXTERN win_T	*prevwin INIT(= NULL);	// previous window
+EXTERN win_T	*prevwin INIT(= NULL);	// previous window (may equal curwin)
 #define ONE_WINDOW (firstwin == lastwin)
 #define W_NEXT(wp) ((wp)->w_next)
 

src/testdir/test_window_cmd.vim

@@ -131,64 +131,6 @@ func Test_window_quit()
   bw Xa Xb
 endfunc
 
-func Test_window_curwin_not_prevwin()
-  botright split
-  call assert_equal(2, winnr())
-  call assert_equal(1, winnr('#'))
-  quit
-  call assert_equal(1, winnr())
-  call assert_equal(0, winnr('#'))
-
-  botright split
-  botright split
-  call assert_equal(3, winnr())
-  call assert_equal(2, winnr('#'))
-  1quit
-  call assert_equal(2, winnr())
-  call assert_equal(1, winnr('#'))
-
-  botright split
-  call assert_equal(1, tabpagenr())
-  call assert_equal(3, winnr())
-  call assert_equal(2, winnr('#'))
-  wincmd T
-  call assert_equal(2, tabpagenr())
-  call assert_equal(1, winnr())
-  call assert_equal(0, winnr('#'))
-  tabfirst
-  call assert_equal(1, tabpagenr())
-  call assert_equal(2, winnr())
-  call assert_equal(0, winnr('#'))
-
-  tabonly
-  botright split
-  wincmd t
-  wincmd p
-  call assert_equal(3, winnr())
-  call assert_equal(1, winnr('#'))
-  quit
-  call assert_equal(2, winnr())
-  call assert_equal(1, winnr('#'))
-
-  botright split
-  wincmd t
-  wincmd p
-  call assert_equal(1, tabpagenr())
-  call assert_equal(3, winnr())
-  call assert_equal(1, winnr('#'))
-  wincmd T
-  call assert_equal(2, tabpagenr())
-  call assert_equal(1, winnr())
-  call assert_equal(0, winnr('#'))
-  tabfirst
-  call assert_equal(1, tabpagenr())
-  call assert_equal(2, winnr())
-  call assert_equal(1, winnr('#'))
-
-  tabonly
-  only
-endfunc
-
 func Test_window_horizontal_split()
   call assert_equal(1, winnr('$'))
   3wincmd s

src/version.c

@@ -704,6 +704,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    170,
 /**/
     169,
 /**/

src/window.c

@@ -5526,15 +5526,11 @@ win_enter_ext(win_T *wp, int flags)
     // may have to copy the buffer options when 'cpo' contains 'S'
     if (wp->w_buffer != curbuf)
 	buf_copy_options(wp->w_buffer, BCO_ENTER | BCO_NOHELP);
-
     if (curwin_invalid == 0)
     {
 	prevwin = curwin;	// remember for CTRL-W p
 	curwin->w_redr_status = TRUE;
     }
-    else if (wp == prevwin)
-	prevwin = NULL;		// don't want it to be the new curwin
-
     curwin = wp;
     curbuf = wp->w_buffer;
     check_cursor();