vim-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 vim/vim#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: vim/vim#14047
closes: vim/vim#14186

https://github.com/vim/vim/commit/d64801e913314d2e19dbb38f60e6d285238debff

3 files changed, 4 insertions, 6 deletions

diff --git a/src/nvim/eval.lua b/src/nvim/eval.lua
index febd022254..73715e2631 100644
--- a/src/nvim/eval.lua
+++ b/src/nvim/eval.lua
@@ -12856,7 +12856,9 @@ M.funcs = {
       	#	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
diff --git a/src/nvim/globals.h b/src/nvim/globals.h
index aecb9d1116..06fb95b577 100644
--- a/src/nvim/globals.h
+++ b/src/nvim/globals.h
@@ -363,7 +363,7 @@ EXTERN bool sys_menu INIT( = false);
 // currently active window.
 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 FOR_ALL_FRAMES(frp, first_frame) \
   for ((frp) = first_frame; (frp) != NULL; (frp) = (frp)->fr_next)
diff --git a/src/nvim/window.c b/src/nvim/window.c
index 521699f2f0..843cf154a7 100644
--- a/src/nvim/window.c
+++ b/src/nvim/window.c
@@ -4915,14 +4915,10 @@ static void win_enter_ext(win_T *const wp, const int flags)
   if (wp->w_buffer != curbuf) {
     buf_copy_options(wp->w_buffer, BCO_ENTER | BCO_NOHELP);
   }
-
   if (!curwin_invalid) {
     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;