fix(grid): double grid_line_start() with ext_messages #31292

Problem:  Hit double grid_line_start() assert when redrawing from
          ext_messages msg_ruler event.
Solution: Do not start() batched grid calls when win_redr_ruler() will not
          puts() anything.

src/nvim/drawscreen.c

@@ -1092,10 +1092,14 @@ int showmode(void)
   win_T *ruler_win = curwin->w_status_height == 0 ? curwin : lastwin_nofloating();
   if (redrawing() && ruler_win->w_status_height == 0 && global_stl_height() == 0
       && !(p_ch == 0 && !ui_has(kUIMessages))) {
+    if (!ui_has(kUIMessages)) {
       grid_line_start(&msg_grid_adj, Rows - 1);
+    }
     win_redr_ruler(ruler_win);
+    if (!ui_has(kUIMessages)) {
       grid_line_flush();
     }
+  }
 
   redraw_cmdline = false;
   redraw_mode = false;

test/functional/ui/messages_spec.lua

@@ -1159,6 +1159,21 @@ stack traceback:
     exec_lua([[vim.print({ foo = "bar" })]])
     screen:expect_unchanged()
   end)
+
+  it('ruler redraw does not crash due to double grid_line_start()', function()
+    exec_lua([[
+      local ns = vim.api.nvim_create_namespace('')
+      vim.ui_attach(ns, { ext_messages = true }, function(event, ...)
+        if event == 'msg_ruler' then
+          vim.api.nvim__redraw({ flush = true })
+        end
+      end)
+      vim.o.ruler = true
+      vim.o.laststatus = 0
+    ]])
+    feed('i')
+    n.assert_alive()
+  end)
 end)
 
 describe('ui/builtin messages', function()