fix: set cursorlineopt=number in terminal mode (#15493)

When entering terminal mode, cursorlineopt is no longer entirely
disabled. Instead, it's set to `number`. Doing so ensures that users
using `set cursorline` combined with `set cursorlineopt=number` have
consistent highlighting of the line numbers, instead of this being
disabled when entering terminal mode.

Co-authored-by: Gregory Anders <greg@gpanders.com>
Co-authored-by: Sean Dewar <seandewar@users.noreply.github.com>

runtime/doc/nvim_terminal_emulator.txt

@@ -53,7 +53,7 @@ next key is sent unless it is <C-N>. Use <C-\><C-N> to return to normal-mode.
 
 Terminal-mode forces these local options:
 
-    'nocursorline'
+    'cursorlineopt' = number
     'nocursorcolumn'
     'scrolloff' = 0
     'sidescrolloff' = 0

src/nvim/terminal.c

@@ -358,11 +358,21 @@ void terminal_enter(void)
   // Disable these options in terminal-mode. They are nonsense because cursor is
   // placed at end of buffer to "follow" output. #11072
   win_T *save_curwin = curwin;
-  int save_w_p_cul = curwin->w_p_cul;
+  bool save_w_p_cul = curwin->w_p_cul;
+  char_u *save_w_p_culopt = NULL;
+  char_u save_w_p_culopt_flags = curwin->w_p_culopt_flags;
   int save_w_p_cuc = curwin->w_p_cuc;
   long save_w_p_so = curwin->w_p_so;
   long save_w_p_siso = curwin->w_p_siso;
+  if (curwin->w_p_cul && curwin->w_p_culopt_flags & CULOPT_NBR) {
+    if (strcmp((char *)curwin->w_p_culopt, "number")) {
+      save_w_p_culopt = curwin->w_p_culopt;
+      curwin->w_p_culopt = (char_u *)xstrdup("number");
+    }
+    curwin->w_p_culopt_flags = CULOPT_NBR;
+  } else {
     curwin->w_p_cul = false;
+  }
   curwin->w_p_cuc = false;
   curwin->w_p_so = 0;
   curwin->w_p_siso = 0;
@@ -386,9 +396,16 @@ void terminal_enter(void)
 
   if (save_curwin == curwin) {  // save_curwin may be invalid (window closed)!
     curwin->w_p_cul = save_w_p_cul;
+    if (save_w_p_culopt) {
+      xfree(curwin->w_p_culopt);
+      curwin->w_p_culopt = save_w_p_culopt;
+    }
+    curwin->w_p_culopt_flags = save_w_p_culopt_flags;
     curwin->w_p_cuc = save_w_p_cuc;
     curwin->w_p_so = save_w_p_so;
     curwin->w_p_siso = save_w_p_siso;
+  } else if (save_w_p_culopt) {
+    xfree(save_w_p_culopt);
   }
 
   // draw the unfocused cursor

test/functional/terminal/buffer_spec.lua

@@ -22,14 +22,28 @@ describe(':terminal buffer', function()
 
   it('terminal-mode forces various options', function()
     feed([[<C-\><C-N>]])
-    command('setlocal cursorline cursorcolumn scrolloff=4 sidescrolloff=7')
+    command('setlocal cursorline cursorlineopt=both cursorcolumn scrolloff=4 sidescrolloff=7')
-    eq({ 1, 1, 4, 7 }, eval('[&l:cursorline, &l:cursorcolumn, &l:scrolloff, &l:sidescrolloff]'))
+    eq({ 'both', 1, 1, 4, 7 }, eval('[&l:cursorlineopt, &l:cursorline, &l:cursorcolumn, &l:scrolloff, &l:sidescrolloff]'))
     eq('n', eval('mode()'))
 
     -- Enter terminal-mode ("insert" mode in :terminal).
     feed('i')
     eq('t', eval('mode()'))
-    eq({ 0, 0, 0, 0 }, eval('[&l:cursorline, &l:cursorcolumn, &l:scrolloff, &l:sidescrolloff]'))
+    eq({ 'number', 1, 0, 0, 0 }, eval('[&l:cursorlineopt, &l:cursorline, &l:cursorcolumn, &l:scrolloff, &l:sidescrolloff]'))
+  end)
+
+  it('terminal-mode does not change cursorlineopt if cursorline is disabled', function()
+    feed([[<C-\><C-N>]])
+    command('setlocal nocursorline cursorlineopt=both')
+    feed('i')
+    eq({ 0, 'both' }, eval('[&l:cursorline, &l:cursorlineopt]'))
+  end)
+
+  it('terminal-mode disables cursorline when cursorlineopt is only set to "line', function()
+    feed([[<C-\><C-N>]])
+    command('setlocal cursorline cursorlineopt=line')
+    feed('i')
+    eq({ 0, 'line' }, eval('[&l:cursorline, &l:cursorlineopt]'))
   end)
 
   describe('when a new file is edited', function()