vim-patch:9.1.0047: issues with temp curwin/buf while cmdwin is open

Problem:  Things that temporarily change/restore curwin/buf (e.g:
          win_execute, some autocmds) may break assumptions that
          curwin/buf is the cmdwin when "cmdwin_type != 0", causing
          issues.

Solution: Expose the cmdwin's real win/buf and check that instead. Also
          try to ensure these variables are NULL if "cmdwin_type == 0",
          allowing them to be used directly in most cases without
          checking cmdwin_type. (Sean Dewar)

Reset and save `cmdwin_old_curwin` in a similar fashion.
Apply suitable changes for API functions and add Lua tests.

988f74311c

test/functional/vimscript/api_functions_spec.lua

@@ -3,6 +3,7 @@ local Screen = require('test.functional.ui.screen')
 local neq, eq, command = helpers.neq, helpers.eq, helpers.command
 local clear = helpers.clear
 local exc_exec, expect, eval = helpers.exc_exec, helpers.expect, helpers.eval
+local exec_lua = helpers.exec_lua
 local insert, pcall_err = helpers.insert, helpers.pcall_err
 local matches = helpers.matches
 local api = helpers.api
@@ -106,6 +107,19 @@ describe('eval-API', function()
       pcall_err(api.nvim_open_term, 0, {})
     )
 
+    matches(
+      'E11: Invalid in command%-line window; <CR> executes, CTRL%-C quits$',
+      pcall_err(
+        exec_lua,
+        [[
+         local cmdwin_buf = vim.api.nvim_get_current_buf()
+         vim.api.nvim_buf_call(vim.api.nvim_create_buf(false, true), function()
+           vim.api.nvim_open_term(cmdwin_buf, {})
+         end)
+       ]]
+      )
+    )
+
     -- But turning a different buffer into a terminal from the cmdwin is OK.
     local term_buf = api.nvim_create_buf(false, true)
     api.nvim_open_term(term_buf, {})