vim-patch:9.0.0190: the way 'cmdheight' can be made zero is inconsistent

Problem:    The way 'cmdheight' can be made zero is inconsistent.
Solution:   Only make 'cmdheight' zero when setting it explicitly, not when
            resizing windows. (closes vim/vim#10890)
f797e309ca

src/nvim/testdir/test_cmdline.vim

@@ -126,6 +126,35 @@ func Test_wildmenu_screendump()
   call delete('XTest_wildmenu')
 endfunc
 
+func Test_changing_cmdheight()
+  CheckScreendump
+
+  let lines =<< trim END
+      set cmdheight=1 laststatus=2
+  END
+  call writefile(lines, 'XTest_cmdheight')
+
+  let buf = RunVimInTerminal('-S XTest_cmdheight', {'rows': 8})
+  call term_sendkeys(buf, ":resize -3\<CR>")
+  call VerifyScreenDump(buf, 'Test_changing_cmdheight_1', {})
+
+  " using the space available doesn't change the status line
+  call term_sendkeys(buf, ":set cmdheight+=3\<CR>")
+  call VerifyScreenDump(buf, 'Test_changing_cmdheight_2', {})
+
+  " using more space moves the status line up
+  call term_sendkeys(buf, ":set cmdheight+=1\<CR>")
+  call VerifyScreenDump(buf, 'Test_changing_cmdheight_3', {})
+
+  " reducing cmdheight moves status line down
+  call term_sendkeys(buf, ":set cmdheight-=2\<CR>")
+  call VerifyScreenDump(buf, 'Test_changing_cmdheight_4', {})
+
+  " clean up
+  call StopVimInTerminal(buf)
+  call delete('XTest_cmdheight')
+endfunc
+
 func Test_map_completion()
   if !has('cmdline_compl')
     return

src/nvim/window.c

@@ -5552,7 +5552,7 @@ static void frame_setheight(frame_T *curfrp, int height)
   }
 
   if (curfrp->fr_parent == NULL) {
-    // topframe: can only change the command line
+    // topframe: can only change the command line height
     if (height > ROWS_AVAIL) {
       // If height is greater than the available space, try to create space for
       // the frame by reducing 'cmdheight' if possible, while making sure
@@ -5915,6 +5915,13 @@ void win_drag_status_line(win_T *dragwin, int offset)
   int row;
   bool up;                      // if true, drag status line up, otherwise down
   int n;
+  static bool p_ch_was_zero = false;
+
+  // If the user explicitly set 'cmdheight' to zero, then allow for dragging
+  // the status line making it zero again.
+  if (p_ch == 0) {
+    p_ch_was_zero = true;
+  }
 
   fr = dragwin->w_frame;
   curfr = fr;
@@ -5965,6 +5972,8 @@ void win_drag_status_line(win_T *dragwin, int offset)
     room = Rows - cmdline_row;
     if (curfr->fr_next != NULL) {
       room -= (int)p_ch + global_stl_height();
+    } else if (!p_ch_was_zero) {
+      room--;
     }
     if (room < 0) {
       room = 0;
@@ -6020,7 +6029,7 @@ void win_drag_status_line(win_T *dragwin, int offset)
     clear_cmdline = true;
   }
   cmdline_row = row;
-  p_ch = MAX(Rows - cmdline_row, 0);
+  p_ch = MAX(Rows - cmdline_row, p_ch_was_zero ? 0 : 1);
   curtab->tp_ch_used = p_ch;
   redraw_all_later(SOME_VALID);
   showmode();
@@ -6384,6 +6393,12 @@ void command_height(void)
   // p_ch was changed in another tab page.
   curtab->tp_ch_used = p_ch;
 
+  // If the space for the command line is already more than 'cmdheight' there
+  // is nothing to do (window size must have decreased).
+  if (p_ch > old_p_ch && cmdline_row <= Rows - p_ch) {
+    return;
+  }
+
   // Find bottom frame with width of screen.
   frp = lastwin_nofloating()->w_frame;
   while (frp->fr_width != Columns && frp->fr_parent != NULL) {

test/functional/ui/statusline_spec.lua

@@ -394,7 +394,7 @@ describe('global statusline', function()
     meths.input_mouse('left', 'drag', '', 0, 14, 10)
     eq(1, meths.get_option('cmdheight'))
     meths.input_mouse('left', 'drag', '', 0, 15, 10)
-    eq(0, meths.get_option('cmdheight'))
+    eq(1, meths.get_option('cmdheight'))
     meths.input_mouse('left', 'drag', '', 0, 14, 10)
     eq(1, meths.get_option('cmdheight'))
   end)