vim-patch:9.0.0670: no space for command line when there is a tabline (#20512)

Problem:    No space for command line when there is a tabline.
Solution:   Correct computation of where the command line should be.
            (closes vim/vim#11295)
c9f5f73206

src/nvim/option.c

@@ -2468,7 +2468,9 @@ static char *set_num_option(int opt_idx, char_u *varp, long value, char *errbuf,
     // if p_ch changed value, change the command line height
     // Only compute the new window layout when startup has been
     // completed. Otherwise the frame sizes may be wrong.
-    if ((p_ch != old_value || topframe->fr_height != Rows - p_ch) && full_screen) {
+    if ((p_ch != old_value
+         || tabline_height() + topframe->fr_height != Rows - p_ch)
+        && full_screen) {
       command_height();
     }
   } else if (pp == &p_uc) {

src/nvim/testdir/test_cmdline.vim

@@ -225,6 +225,16 @@ func Test_changing_cmdheight()
   call StopVimInTerminal(buf)
 endfunc
 
+func Test_cmdheight_tabline()
+  CheckScreendump
+
+  let buf = RunVimInTerminal('-c "set ls=2" -c "set stal=2" -c "set cmdheight=1"', {'rows': 6})
+  call VerifyScreenDump(buf, 'Test_cmdheight_tabline_1', {})
+
+  " clean up
+  call StopVimInTerminal(buf)
+endfunc
+
 func Test_map_completion()
   if !has('cmdline_compl')
     return

src/nvim/window.c

@@ -4300,7 +4300,7 @@ static void enter_tabpage(tabpage_T *tp, buf_T *old_curbuf, bool trigger_enter_a
   // When cmdheight is changed in a tab page with '<C-w>-', cmdline_row is
   // changed but p_ch and tp_ch_used are not changed. Thus we also need to
   // check cmdline_row.
-  if ((row < cmdline_row) && (cmdline_row <= Rows - p_ch)) {
+  if (row < cmdline_row && cmdline_row <= Rows - p_ch) {
     clear_cmdline = true;
   }
 
@@ -6409,7 +6409,7 @@ void command_height(void)
   curtab->tp_ch_used = p_ch;
 
   // Update cmdline_row to what it should be: just below the last window.
-  cmdline_row = topframe->fr_height;
+  cmdline_row = topframe->fr_height + tabline_height();
 
   // If cmdline_row is smaller than what it is supposed to be for 'cmdheight'
   // then set old_p_ch to what it would be, so that the windows get resized