vim-patch:9.2.0985: Multiline messages not visible when mapping starts cmdline (#41405)

Problem:  Multiline messages exceeding 'cmdheight' not visible when a
          mapping starts cmdline immediately after it (after 9.2.0967).
Solution: Revert patch 9.2.0967 and use a different solution (zeertzjq).

fixes:  vim/vim#21098
closes: vim/vim#21101

fb4866a2dd

test/functional/legacy/messages_spec.lua

@@ -889,15 +889,15 @@ describe('messages', function()
     ]])
   end)
 
-  -- oldtest: Test_hit_enter_no_eat_mapped_keys()
-  it('hit-enter prompt does not eat keys from a mapping', function()
+  -- oldtest: Test_hit_enter_during_mapping()
+  it('hit-enter prompt during a mapping', function()
     screen = Screen.new(75, 10)
     exec([[
-      set ruler more
+      set ruler
       call setline(1, range(1, 20))
-      " The 8-line :echo scrolls the screen and would raise a hit-enter prompt;
-      " the mapping then runs "gg" to move the cursor to line 1.
+      " The 8-line :echo leads to a hit-enter prompt.
       nnoremap X :echo "a\nb\nc\nd\ne\nf\ng\nh"<CR>gg
+      nnoremap \b :echo "a\nb\nc\nd\ne\nf\ng\nh"<CR>:b<Space>
       normal! 10G
     ]])
     t.eq({ mode = 'n', blocking = false }, api.nvim_get_mode())
@@ -908,6 +908,22 @@ describe('messages', function()
     -- cursor stays put.  With the fix "gg" runs and moves the cursor to line 1.
     t.eq({ mode = 'n', blocking = false }, api.nvim_get_mode())
     t.eq({ 1, 0 }, api.nvim_win_get_cursor(0))
+
+    -- If a mapping starts cmdline after multiline messages exceeding 'cmdheight',
+    -- the messages should still be visible.
+    feed('\\b')
+    screen:expect([[
+      {3:                                                                           }|
+      a                                                                          |
+      b                                                                          |
+      c                                                                          |
+      d                                                                          |
+      e                                                                          |
+      f                                                                          |
+      g                                                                          |
+      h                                                                          |
+      :b ^                                                                        |
+    ]])
   end)
 
   -- oldtest: Test_fileinfo_after_last_bd()