fix(messages): allow more prompt in headless mode with UI (#27905)

Problem:  More prompt is not shown in headless mode even if there is a
          UI attached.
Solution: Don't skip more prompt when there is a UI active.

src/nvim/message.c

@@ -2713,7 +2713,7 @@ static bool do_more_prompt(int typed_char)
   // If headless mode is enabled and no input is required, this variable
   // will be true. However If server mode is enabled, the message "--more--"
   // should be displayed.
-  bool no_need_more = headless_mode && !embedded_mode;
+  bool no_need_more = headless_mode && !embedded_mode && !ui_active();
 
   // We get called recursively when a timer callback outputs a message. In
   // that case don't show another prompt. Also when at the hit-Enter prompt

test/functional/treesitter/highlight_spec.lua

@@ -775,18 +775,23 @@ describe('treesitter highlighting (C)', function()
         declarator: (pointer_declarator) @variable.parameter)
     ]]
 
-    exec_lua([[
+    exec_lua(
+      [[
       local query = ...
       vim.treesitter.query.set('c', 'highlights', query)
       vim.treesitter.highlighter.new(vim.treesitter.get_parser(0, 'c'))
-    ]], query)
+    ]],
+      query
+    )
 
-    screen:expect{grid=[[
+    screen:expect {
+      grid = [[
         void foo(int {4:*}{11:bar});                                            |
       ^                                                                 |
       {1:~                                                                }|*15
                                                                        |
-    ]]}
+    ]],
+    }
   end)
 end)
 

test/functional/ui/messages_spec.lua

@@ -3,6 +3,7 @@ local Screen = require('test.functional.ui.screen')
 local clear, feed = helpers.clear, helpers.feed
 local eval = helpers.eval
 local eq = helpers.eq
+local neq = helpers.neq
 local command = helpers.command
 local set_method_error = helpers.set_method_error
 local api = helpers.api
@@ -15,6 +16,7 @@ local exc_exec = helpers.exc_exec
 local exec_lua = helpers.exec_lua
 local poke_eventloop = helpers.poke_eventloop
 local assert_alive = helpers.assert_alive
+local retry = helpers.retry
 local is_os = helpers.is_os
 local is_ci = helpers.is_ci
 local fn = helpers.fn
@@ -2497,3 +2499,47 @@ aliquip ex ea commodo consequat.]]
     }
   end)
 end)
+
+it('pager works in headless mode with UI attached', function()
+  skip(is_os('win'))
+  clear()
+  local child_server = assert(helpers.new_pipename())
+  fn.jobstart({ nvim_prog, '--clean', '--headless', '--listen', child_server })
+  retry(nil, nil, function()
+    neq(nil, vim.uv.fs_stat(child_server))
+  end)
+
+  local child_session = helpers.connect(child_server)
+  local child_screen = Screen.new(40, 6)
+  child_screen:attach(nil, child_session)
+
+  child_session:notify('nvim_command', [[echo range(100)->join("\n")]])
+  child_screen:expect([[
+    0                                       |
+    1                                       |
+    2                                       |
+    3                                       |
+    4                                       |
+    -- More --^                              |
+  ]])
+
+  child_session:request('nvim_input', 'G')
+  child_screen:expect([[
+    95                                      |
+    96                                      |
+    97                                      |
+    98                                      |
+    99                                      |
+    Press ENTER or type command to continue^ |
+  ]])
+
+  child_session:request('nvim_input', 'g')
+  child_screen:expect([[
+    0                                       |
+    1                                       |
+    2                                       |
+    3                                       |
+    4                                       |
+    -- More --^                              |
+  ]])
+end)