patch 9.1.0001: when closing window, wincmd p may fail

Avoid `prevwin == curwin` when closing `curwin`

Problem:  When closing the current window (or when moving it to a tabpage), the
          previous window may refer to the new current window
          (`winnr() == winnr('#')`) if that window is selected as the
          new current window.

Solution: Set `prevwin = NULL` when switching away from an invalid `curwin` and
          the target window was the `prevwin`.
          (Sean Dewar)

related: #4537
closes: #13762

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

src/testdir/test_window_cmd.vim

@@ -131,6 +131,64 @@ 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 */
+/**/
+    1,
 /**/
     0
 };

src/window.c

@@ -5381,11 +5381,15 @@ 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();