aucmd_prepbuf: also restore `prevwin` #9741

bisected to f5d5da391715

Other test steps:

    nvim -u NORC
    :terminal tree /    " Produces lots of output
    :edit somefile.txt
    :vsplit
    :vsplit
    <c-w>l
    <c-w>l
    <c-w>h
    <c-w>p

4 files changed, 55 insertions, 35 deletions

diff --git a/src/nvim/buffer_defs.h b/src/nvim/buffer_defs.h
index 48cef9b1e7..d35e3b49d8 100644
--- a/src/nvim/buffer_defs.h
+++ b/src/nvim/buffer_defs.h
@@ -987,18 +987,16 @@ typedef struct {
                                           .relative = 0, .external = false, \
                                           .focusable = true })
 
-/*
- * Structure which contains all information that belongs to a window
- *
- * All row numbers are relative to the start of the window, except w_winrow.
- */
+/// Structure which contains all information that belongs to a window.
+///
+/// All row numbers are relative to the start of the window, except w_winrow.
 struct window_S {
   handle_T handle;                  ///< unique identifier for the window
 
   buf_T       *w_buffer;            ///< buffer we are a window into (used
                                     ///< often, keep it the first item!)
 
-  synblock_T  *w_s;                 /* for :ownsyntax */
+  synblock_T  *w_s;                 ///< for :ownsyntax
 
   int w_hl_id_normal;               ///< 'winhighlight' normal id
   int w_hl_attr_normal;             ///< 'winhighlight' normal final attrs
@@ -1008,18 +1006,18 @@ struct window_S {
 
   int w_hl_needs_update;            ///< attrs need to be recalculated
 
-  win_T       *w_prev;              /* link to previous window */
-  win_T       *w_next;              /* link to next window */
-  bool w_closing;                   /* window is being closed, don't let
-                                       autocommands close it too. */
+  win_T       *w_prev;              ///< link to previous window
+  win_T       *w_next;              ///< link to next window
+  bool w_closing;                   ///< window is being closed, don't let
+                                    ///  autocommands close it too.
 
-  frame_T     *w_frame;             /* frame containing this window */
+  frame_T     *w_frame;             ///< frame containing this window
 
-  pos_T w_cursor;                   /* cursor position in buffer */
+  pos_T w_cursor;                   ///< cursor position in buffer
 
-  colnr_T w_curswant;               /* The column we'd like to be at.  This is
-                                       used to try to stay in the same column
-                                       for up/down cursor motions. */
+  colnr_T w_curswant;               ///< Column we want to be at.  This is
+                                    ///  used to try to stay in the same column
+                                    ///  for up/down cursor motions.
 
   int w_set_curswant;               // If set, then update w_curswant the next
                                     // time through cursupdate() to the
diff --git a/src/nvim/fileio.c b/src/nvim/fileio.c
index 7be4107c94..53b945d983 100644
--- a/src/nvim/fileio.c
+++ b/src/nvim/fileio.c
@@ -6527,6 +6527,7 @@ aucmd_prepbuf (
     win = curwin;
 
   aco->save_curwin = curwin;
+  aco->save_prevwin = prevwin;
   aco->save_curbuf = curbuf;
   if (win != NULL) {
     /* There is a window for "buf" in the current tab page, make it the
@@ -6624,6 +6625,8 @@ win_found:
       // Hmm, original window disappeared.  Just use the first one.
       curwin = firstwin;
     }
+    prevwin = win_valid(aco->save_prevwin) ? aco->save_prevwin
+              : firstwin;  // window disappeared?
     vars_clear(&aucmd_win->w_vars->dv_hashtab);      // free all w: variables
     hash_init(&aucmd_win->w_vars->dv_hashtab);       // re-use the hashtab
     curbuf = curwin->w_buffer;
@@ -6656,6 +6659,8 @@ win_found:
       }
 
       curwin = aco->save_curwin;
+      prevwin = win_valid(aco->save_prevwin) ? aco->save_prevwin
+                : firstwin;  // window disappeared?
       curbuf = curwin->w_buffer;
       // In case the autocommand moves the cursor to a position that does not
       // exist in curbuf
diff --git a/src/nvim/fileio.h b/src/nvim/fileio.h
index 8db4b89806..a6011ec414 100644
--- a/src/nvim/fileio.h
+++ b/src/nvim/fileio.h
@@ -23,6 +23,7 @@ typedef struct {
   buf_T       *save_curbuf;     ///< saved curbuf
   int use_aucmd_win;            ///< using aucmd_win
   win_T       *save_curwin;     ///< saved curwin
+  win_T       *save_prevwin;    ///< saved prevwin
   win_T       *new_curwin;      ///< new curwin
   bufref_T     new_curbuf;      ///< new curbuf
   char_u      *globaldir;       ///< saved value of globaldir
diff --git a/test/functional/api/buffer_spec.lua b/test/functional/api/buffer_spec.lua
index 93599c04f1..d26cb0dbb3 100644
--- a/test/functional/api/buffer_spec.lua
+++ b/test/functional/api/buffer_spec.lua
@@ -24,8 +24,8 @@ describe('api/buf', function()
   end
 
 
-  describe('line_count, insert and del_line', function()
-    it('works', function()
+  describe('nvim_buf_set_lines, nvim_buf_line_count', function()
+    it('deprecated forms', function()
       eq(1, curbuf_depr('line_count'))
       curbuf_depr('insert', -1, {'line'})
       eq(2, curbuf_depr('line_count'))
@@ -70,7 +70,7 @@ describe('api/buf', function()
     end)
   end)
 
-  describe('{get,set,del}_line', function()
+  describe('deprecated: {get,set,del}_line', function()
     it('works', function()
       eq('', curbuf_depr('get_line', 0))
       curbuf_depr('set_line', 0, 'line1')
@@ -102,7 +102,7 @@ describe('api/buf', function()
     end)
   end)
 
-  describe('{get,set}_line_slice', function()
+  describe('deprecated: {get,set}_line_slice', function()
     it('get_line_slice: out-of-bounds returns empty array', function()
       curbuf_depr('set_line_slice', 0, 0, true, true, {'a', 'b', 'c'})
       eq({'a', 'b', 'c'}, curbuf_depr('get_line_slice', 0, 2, true, true)) --sanity
@@ -149,7 +149,7 @@ describe('api/buf', function()
     end)
   end)
 
-  describe('{get,set}_lines', function()
+  describe('nvim_buf_get_lines, nvim_buf_set_lines', function()
     local get_lines, set_lines = curbufmeths.get_lines, curbufmeths.set_lines
     local line_count = curbufmeths.line_count
 
@@ -272,7 +272,7 @@ describe('api/buf', function()
       eq({}, get_lines(-3, -4, true))
     end)
 
-    it('set_line_slice: out-of-bounds can extend past end', function()
+    it('set_lines: out-of-bounds can extend past end', function()
       set_lines(0, -1, true, {'a', 'b', 'c'})
       eq({'a', 'b', 'c'}, get_lines(0, -1, true)) --sanity
 
@@ -286,7 +286,7 @@ describe('api/buf', function()
       eq({'e', 'a', 'b', 'c', 'd'}, get_lines(0, -1, true))
     end)
 
-    it("set_line on alternate buffer does not access invalid line (E315)", function()
+    it("set_lines on alternate buffer does not access invalid line (E315)", function()
       feed_command('set hidden')
       insert('Initial file')
       command('enew')
@@ -334,9 +334,27 @@ describe('api/buf', function()
         {2:-- INSERT --}        |
       ]])
     end)
+
+    it('set_lines on hidden buffer preserves "previous window" #9741', function()
+      insert([[
+        visible buffer line 1
+        line 2
+      ]])
+      local hiddenbuf = meths.create_buf(false,true)
+      command('vsplit')
+      command('vsplit')
+      feed('<c-w>l<c-w>l<c-w>l')
+      eq(3, funcs.winnr())
+      feed('<c-w>h')
+      eq(2, funcs.winnr())
+      meths.buf_set_lines(hiddenbuf, 0, -1, true,
+                          {'hidden buffer line 1', 'line 2'})
+      feed('<c-w>p')
+      eq(3, funcs.winnr())
+    end)
   end)
 
-  describe('get_offset', function()
+  describe('nvim_buf_get_offset', function()
     local get_offset = curbufmeths.get_offset
     it('works', function()
       curbufmeths.set_lines(0,-1,true,{'Some\r','exa\000mple', '', 'buf\rfer', 'text'})
@@ -373,7 +391,7 @@ describe('api/buf', function()
     end)
   end)
 
-  describe('{get,set,del}_var', function()
+  describe('nvim_buf_get_var, nvim_buf_set_var, nvim_buf_del_var', function()
     it('works', function()
       curbuf('set_var', 'lua', {1, 2, {['3'] = 1}})
       eq({1, 2, {['3'] = 1}}, curbuf('get_var', 'lua'))
@@ -393,7 +411,7 @@ describe('api/buf', function()
     end)
   end)
 
-  describe('get_changedtick', function()
+  describe('nvim_buf_get_changedtick', function()
     it('works', function()
       eq(2, curbufmeths.get_changedtick())
       curbufmeths.set_lines(0, 1, false, {'abc\0', '\0def', 'ghi'})
@@ -417,7 +435,7 @@ describe('api/buf', function()
     end)
   end)
 
-  describe('{get,set}_option', function()
+  describe('nvim_buf_get_option, nvim_buf_set_option', function()
     it('works', function()
       eq(8, curbuf('get_option', 'shiftwidth'))
       curbuf('set_option', 'shiftwidth', 4)
@@ -430,7 +448,7 @@ describe('api/buf', function()
     end)
   end)
 
-  describe('{get,set}_name', function()
+  describe('nvim_buf_get_name, nvim_buf_set_name', function()
     it('works', function()
       nvim('command', 'new')
       eq('', curbuf('get_name'))
@@ -438,14 +456,12 @@ describe('api/buf', function()
       curbuf('set_name', new_name)
       eq(new_name, curbuf('get_name'))
       nvim('command', 'w!')
-      local f = io.open(new_name)
-      ok(f ~= nil)
-      f:close()
+      eq(1, funcs.filereadable(new_name))
       os.remove(new_name)
     end)
   end)
 
-  describe('is_loaded', function()
+  describe('nvim_buf_is_loaded', function()
     it('works', function()
       -- record our buffer number for when we unload it
       local bufnr = curbuf('get_number')
@@ -470,7 +486,7 @@ describe('api/buf', function()
     end)
   end)
 
-  describe('is_valid', function()
+  describe('nvim_buf_is_valid', function()
     it('works', function()
       nvim('command', 'new')
       local b = nvim('get_current_buf')
@@ -480,12 +496,12 @@ describe('api/buf', function()
     end)
   end)
 
-  describe('get_mark', function()
+  describe('nvim_buf_get_mark', function()
     it('works', function()
       curbuf('set_lines', -1, -1, true, {'a', 'bit of', 'text'})
       curwin('set_cursor', {3, 4})
-      nvim('command', 'mark V')
-      eq({3, 0}, curbuf('get_mark', 'V'))
+      nvim('command', 'mark v')
+      eq({3, 0}, curbuf('get_mark', 'v'))
     end)
   end)
 end)