fix(wininfo): when freeing windows, free the lowest priority wininfo

On master (and also before #31539) closing a window could cause the
used wininfo for a buffer to change. This is due to always removing the
previous NULL wininfo when deleting a window, even if that wininfo had
higher priority than the the deleted window's own wininfo.

Instead delete the wininfo with lowest priority. This retains the memory
saving efect while not affecting the effective value of window options
and so on.

2 files changed, 49 insertions, 4 deletions

diff --git a/src/nvim/window.c b/src/nvim/window.c
index b1c483547c..190ac500b0 100644
--- a/src/nvim/window.c
+++ b/src/nvim/window.c
@@ -5242,11 +5242,13 @@ void win_free(win_T *wp, tabpage_T *tp)
   // freed memory is re-used for another window.
   FOR_ALL_BUFFERS(buf) {
     WinInfo *wip_wp = NULL;
+    size_t pos_wip = kv_size(buf->b_wininfo);
     size_t pos_null = kv_size(buf->b_wininfo);
     for (size_t i = 0; i < kv_size(buf->b_wininfo); i++) {
       WinInfo *wip = kv_A(buf->b_wininfo, i);
       if (wip->wi_win == wp) {
         wip_wp = wip;
+        pos_wip = i;
       } else if (wip->wi_win == NULL) {
         pos_null = i;
       }
@@ -5254,11 +5256,12 @@ void win_free(win_T *wp, tabpage_T *tp)
 
     if (wip_wp) {
       wip_wp->wi_win = NULL;
-      // If there already is an entry with "wi_win" set to NULL it
-      // must be removed, it would never be used.
+      // If there already is an entry with "wi_win" set to NULL, only
+      // the first entry with NULL will ever be used, delete the other one.
       if (pos_null < kv_size(buf->b_wininfo)) {
-        free_wininfo(kv_A(buf->b_wininfo, pos_null), buf);
-        kv_shift(buf->b_wininfo, pos_null, 1);
+        size_t pos_delete = MAX(pos_null, pos_wip);
+        free_wininfo(kv_A(buf->b_wininfo, pos_delete), buf);
+        kv_shift(buf->b_wininfo, pos_delete, 1);
       }
     }
   }
diff --git a/test/functional/api/window_spec.lua b/test/functional/api/window_spec.lua
index 92999f383a..5d7dab2a7e 100644
--- a/test/functional/api/window_spec.lua
+++ b/test/functional/api/window_spec.lua
@@ -488,6 +488,48 @@ describe('API/win', function()
       assert_alive()
     end)
 
+    describe('after closing', function()
+      local buf, win0, win1, win2
+
+      before_each(function()
+        win0 = api.nvim_get_current_win()
+        command('new')
+        buf = api.nvim_get_current_buf()
+        win1 = api.nvim_get_current_win()
+        command('set numberwidth=10')
+        command('split')
+        win2 = api.nvim_get_current_win()
+        command('set numberwidth=15')
+        command('enew')
+        api.nvim_set_current_win(win1)
+        command('normal ix')
+        command('enew')
+        api.nvim_set_current_win(win0)
+        eq(4, api.nvim_get_option_value('numberwidth', {}))
+      end)
+
+      -- at this point buffer `buf` is current in no windows. Closing shouldn't affect its defaults
+      it('0 windows', function()
+        api.nvim_set_current_buf(buf)
+        eq(10, api.nvim_get_option_value('numberwidth', {}))
+      end)
+
+      it('1 window', function()
+        api.nvim_win_close(win1, false)
+
+        api.nvim_set_current_buf(buf)
+        eq(10, api.nvim_get_option_value('numberwidth', {}))
+      end)
+
+      it('2 windows', function()
+        api.nvim_win_close(win1, false)
+        api.nvim_win_close(win2, false)
+
+        api.nvim_set_current_buf(buf)
+        eq(10, api.nvim_get_option_value('numberwidth', {}))
+      end)
+    end)
+
     it('returns values for unset local options', function()
       eq(-1, api.nvim_get_option_value('scrolloff', { win = 0, scope = 'local' }))
     end)