fix(autocmd): 'cmdheight' OptionSet with valid window grids (#34619)

Problem:  OptionSet autocmd emitted with invalid grids after entering
          tabpage with different 'cmdheight'.
Solution: First call `tabpage_check_windows()` before changing 'cmdheight'.
          Add a test that exercises the `vim._extui` cmdline.

src/nvim/window.c

@@ -4441,18 +4441,17 @@ static void enter_tabpage(tabpage_T *tp, buf_T *old_curbuf, bool trigger_enter_a
 
   use_tabpage(tp);
 
-  if (p_ch != curtab->tp_ch_used) {
-    // Use the stored value of p_ch, so that it can be different for each tab page.
-    // Handle other side-effects but avoid setting frame sizes, which are still correct.
-    OptInt new_ch = curtab->tp_ch_used;
-    curtab->tp_ch_used = p_ch;
-    command_frame_height = false;
-    set_option_value(kOptCmdheight, NUMBER_OPTVAL(new_ch), 0);
-    command_frame_height = true;
-  }
-
   if (old_curtab != curtab) {
     tabpage_check_windows(old_curtab);
+    if (p_ch != curtab->tp_ch_used) {
+      // Use the stored value of p_ch, so that it can be different for each tab page.
+      // Handle other side-effects but avoid setting frame sizes, which are still correct.
+      OptInt new_ch = curtab->tp_ch_used;
+      curtab->tp_ch_used = p_ch;
+      command_frame_height = false;
+      set_option_value(kOptCmdheight, NUMBER_OPTVAL(new_ch), 0);
+      command_frame_height = true;
+    }
   }
 
   // We would like doing the TabEnter event first, but we don't have a

test/functional/ui/cmdline2_spec.lua

@@ -0,0 +1,46 @@
+-- Tests for (protocol-driven) ui2, intended to replace the legacy message grid UI.
+
+local n = require('test.functional.testnvim')()
+local Screen = require('test.functional.ui.screen')
+
+local clear, exec, exec_lua, feed = n.clear, n.exec, n.exec_lua, n.feed
+
+describe('cmdline2', function()
+  local screen
+  before_each(function()
+    clear()
+    screen = Screen.new()
+    screen:add_extra_attr_ids({
+      [100] = { foreground = Screen.colors.Magenta1, bold = true },
+    })
+    exec_lua(function()
+      require('vim._extui').enable({})
+    end)
+  end)
+
+  it("no crash for invalid grid after 'cmdheight' OptionSet", function()
+    exec('tabnew | tabprev')
+    feed(':set ch=0')
+    screen:expect([[
+      {5: }{100:2}{5: [No Name] }{24: [No Name] }{2:                            }{24:X}|
+                                                           |
+      {1:~                                                    }|*11
+      {16::}{15:set} {16:ch}{15:=}0^                                            |
+    ]])
+    feed('<CR>')
+    exec('tabnext')
+    screen:expect([[
+      {24: [No Name] }{5: }{100:2}{5: [No Name] }{2:                            }{24:X}|
+      ^                                                     |
+      {1:~                                                    }|*11
+      {16::}{15:set} {16:ch}{15:=}0                                            |
+    ]])
+    exec('tabnext')
+    screen:expect([[
+      {5: }{100:2}{5: [No Name] }{24: [No Name] }{2:                            }{24:X}|
+      ^                                                     |
+      {1:~                                                    }|*12
+    ]])
+    n.assert_alive()
+  end)
+end)

test/functional/ui/messages2_spec.lua

@@ -1,3 +1,5 @@
+-- Tests for (protocol-driven) ui2, intended to replace the legacy message grid UI.
+
 local n = require('test.functional.testnvim')()
 local Screen = require('test.functional.ui.screen')