vim-patch:9.1.0512: Mode message for spell completion doesn't match allowed keys (#29437)

Problem:  Mode message for spell completion doesn't match allowed keys
          (Kyle Kovacs)
Solution: Show "^S" instead of "s".
          (zeertzjq)

This matches the code in vim_is_ctrl_x_key():

	case CTRL_X_SPELL:
	    return (c == Ctrl_S || c == Ctrl_P || c == Ctrl_N);

fixes: neovim/neovim#29431
closes: vim/vim#15065

7002c055d5
(cherry picked from commit 1db1476fd9)

src/nvim/insexpand.c

@@ -121,7 +121,7 @@ static char *ctrl_x_msgs[] = {
   N_(" Command-line completion (^V^N^P)"),
   N_(" User defined completion (^U^N^P)"),
   N_(" Omni completion (^O^N^P)"),
-  N_(" Spelling suggestion (s^N^P)"),
+  N_(" Spelling suggestion (^S^N^P)"),
   N_(" Keyword Local completion (^N^P)"),
   NULL,  // CTRL_X_EVAL doesn't use msg.
   N_(" Command-line completion (^V^N^P)"),

test/old/testdir/test_spell.vim

@@ -5,6 +5,7 @@ source check.vim
 CheckFeature spell
 
 source screendump.vim
+source view_util.vim
 
 func TearDown()
   set nospell
@@ -300,6 +301,20 @@ func Test_compl_with_CTRL_X_CTRL_K_using_spell()
   set spell& spelllang& dictionary& ignorecase&
 endfunc
 
+func Test_compl_with_CTRL_X_s()
+  new
+  set spell spelllang=en_us showmode
+  inoremap <buffer><F2> <Cmd>let g:msg = Screenline(&lines)<CR>
+
+  call feedkeys("STheatre\<C-X>s\<F2>\<C-Y>\<Esc>", 'tx')
+  call assert_equal(['Theater'], getline(1, '$'))
+  call assert_match('(^S^N^P)', g:msg)
+
+  bwipe!
+  set spell& spelllang& showmode&
+  unlet g:msg
+endfunc
+
 func Test_spellrepall()
   new
   set spell