feat(cmdwin): implement cmdwin as a normal buf+win

Problem:

cmdwin (the `:q` cmdline buffer) has various limitations which require
special-casing all over the codebase.

Besides complicating the code, it also breaks async plugins if they try
to create buffers/windows after some work is done, if the user happens
to open cmdwin at the wrong the moment:

    Lua callback: …/guh.nvim/lua/guh/util.lua:531:
    E11: Invalid in command-line window; <CR> executes, CTRL-C quits
    stack traceback:
        [C]: in function 'nvim_buf_delete'
        …/guh.nvim/lua/guh/util.lua:531: in function <…/guh.nvim/lua/guh/util.lua:526>

Solution:

Just say no to "inception". Reimplement cmdwin as a normal buffer+window.

All of the cmdwin contortions (in both core, and innocent plugins) exist
literally only to support "inception": recursive
cmdwin-in-cmdline-things, like `<c-r>=`, `/`, search-during-substitute,
`:input()`, etc. So we just won't support that (though I have
a potential plan for that later, which I call "modal parking lot").

The benefit is that plugins, and core, no longer have to care about
cmdwin.

BONUS:
- mouse-drag on vertical separators works (it only worked for
  horizontal/statusline before)
- inccommand-in-cmdwin now works correctly, for free (thus don't need
  #40077).

POTENTIAL FOLLOWUPS
- Drop `CHECK_CMDWIN` ("E11: Invalid in command-line window"), allow chaos.
- Unify `BUFLOCK_OK` / `LOCK_OK` ?

DESIGN:
- Eliminate lots of C globals, `EX_CMDWIN`, etc.
- `text_locked()` no longer reports true for cmdwin.
- cmdwin = a normal window with 'winfixbuf', 'bufhidden=wipe',
  'buftype=nofile'. Invariants come from those options rather than
  special cases throughout the codebase.
- `nv_record` for q:/q//q? calls Lua
  `nlua_call_vimfn("vim._core.cmdwin", …)`. No `K_CMDWIN`
  / cmdline-reader detour.
- `cedit_key` (`c_CTRL-F`) schedules a deferred event that calls
  `vim._core.cmdwin.open(type, content, pos)` and returns `Ctrl_C` so
  the in-flight cmdline cancels. Reader state is not serialized; instead
  the captured `(type, line, col)` is replayed via
  `nvim_feedkeys(type..line.."<CR>", "nt", …)` after user confirms.
- On confirm/cancel: `<CR>` / `<C-C>` calls into Lua which closes the
  window and re-feeds the cmdline.

BREAKING CHANGES:
- Expression-register cmdline (`<C-R>=` from insert-mode) no longer
  supports cmdwin. Same applies to `input()` / `inputlist()` (already
  covered by `text_locked`).
- Usage of cmdwin in macros/mappings will probably break (assuming they
  ever worked).

runtime/lua/vim/_core/cmdwin.lua

@@ -0,0 +1,168 @@
+--- @brief Command-line window (q:, q/, q?, c_CTRL-F).
+---
+--- Implements cmdwin as a normal window+buffer, instead of the legacy nested `state_enter` loop.
+--- - On confirm, curline is fed back through the cmdline via |nvim_feedkeys()|.
+--- - On cancel, curline is pre-filled in the ":" cmdline.
+
+local M = {}
+
+--- @class vim._core.cmdwin.State
+--- @field type string   ':', '/', '?'
+--- @field win integer   cmdwin window id
+--- @field buf integer   cmdwin buffer id
+--- @field caller_win integer  Window to return-to on close
+
+--- @type vim._core.cmdwin.State?
+local state = nil
+
+local cmdwin_types = { [':'] = true, ['/'] = true, ['?'] = true }
+
+--- Fills the cmdwin buffer with the cmdline history.
+local function fill_history(buf, type)
+  local histname = type == ':' and 'cmd' or (type == '/' or type == '?') and 'search' or nil
+  assert(histname, 'cmdwin: unknown type: ' .. tostring(type))
+  local n = vim.fn.histnr(histname)
+  if n <= 0 then -- May be -1 if history is empty.
+    return
+  end
+  local lines = {} --- @type string[]
+  for i = 1, n do
+    local h = vim.fn.histget(histname, i)
+    if h ~= '' then
+      -- One cmdwin line = one cmdline. Entry may have embedded newlines (e.g. via feedkeys or :execute).
+      lines[#lines + 1] = (h:gsub('\n', '\0'))
+    end
+  end
+  if #lines > 0 then
+    vim.api.nvim_buf_set_lines(buf, 0, -1, false, lines)
+  end
+end
+
+--- Open the command-line window.
+---
+--- @param type? string  ':', '/', '?'. Default ':'.
+--- @param init_line? string  Pre-fill the last line (the "live" cmdline).
+--- @param init_col? integer  1-based cursor column in the last line.
+function M.open(type, init_line, init_col)
+  type = type or ':'
+  assert(cmdwin_types[type], 'cmdwin: unknown type: ' .. tostring(type))
+  if state ~= nil then
+    vim.api.nvim_echo(
+      { { 'E1292: Command-line window is already open', 'ErrorMsg' } },
+      true,
+      { err = true }
+    )
+    return
+  end
+
+  local caller = vim.api.nvim_get_current_win()
+
+  -- Split a horizontal window at the bottom, sized by 'cmdwinheight'.
+  local ok, err = pcall(function()
+    vim.cmd(('botright %dnew'):format(vim.o.cmdwinheight))
+  end)
+  if not ok then
+    vim.api.nvim_echo({ { tostring(err), 'ErrorMsg' } }, true, { err = true })
+    return
+  end
+  local win = vim.api.nvim_get_current_win()
+  local buf = vim.api.nvim_get_current_buf()
+
+  vim.bo[buf].buftype = 'nofile'
+  vim.bo[buf].bufhidden = 'wipe'
+  vim.bo[buf].swapfile = false
+  vim.bo[buf].buflisted = false
+  vim.wo[win][0].winfixbuf = true
+  vim.wo[win][0].foldenable = false
+  -- Show cmdwin-char via 'statuscolumn'.
+  vim.wo[win][0].statuscolumn = '%#NonText#' .. type .. ' '
+
+  fill_history(buf, type)
+
+  -- Append the in-flight cmdline (or empty placeholder) as the last line.
+  vim.api.nvim_buf_set_lines(buf, -1, -1, false, { init_line or '' })
+  local last = vim.api.nvim_buf_line_count(buf)
+  vim.api.nvim_win_set_cursor(win, { last, math.max(0, (init_col or 1) - 1) })
+
+  pcall(vim.api.nvim_buf_set_name, buf, '[Command Line]')
+  if type == ':' then
+    vim.bo[buf].filetype = 'vim'
+  end
+
+  vim.api.nvim__cmdwin_set(type, buf) -- Update the C-side globals.
+
+  state = {
+    type = type,
+    win = win,
+    buf = buf,
+    caller_win = caller,
+  }
+
+  -- Buffer-local mappings.
+  vim.keymap.set(
+    { 'n', 'i' },
+    '<CR>',
+    [[<C-\><C-N><Cmd>lua require'vim._core.cmdwin'.confirm()<CR>]],
+    { buffer = buf, silent = true, desc = 'cmdwin: execute' }
+  )
+  vim.keymap.set(
+    { 'n', 'i', 'x' },
+    '<C-C>',
+    [[<C-\><C-N><Cmd>lua require'vim._core.cmdwin'.cancel()<CR>]],
+    { buffer = buf, silent = true, desc = 'cmdwin: cancel' }
+  )
+
+  -- Clean up if the window is closed by other means (`:q`, `:close`, etc.).
+  vim.api.nvim_create_autocmd({ 'WinClosed' }, {
+    buffer = buf,
+    once = true,
+    callback = function()
+      if state ~= nil then
+        M._cleanup()
+      end
+    end,
+  })
+
+  vim.api.nvim_exec_autocmds('CmdwinEnter', { pattern = type, modeline = false })
+end
+
+--- @private
+function M._cleanup()
+  if state == nil then
+    return
+  end
+  local s = state
+  state = nil
+  pcall(vim.api.nvim__cmdwin_set, '', 0) -- Clear the C-side globals.
+  pcall(vim.api.nvim_exec_autocmds, 'CmdwinLeave', { pattern = s.type, modeline = false })
+  if vim.api.nvim_win_is_valid(s.win) then
+    pcall(vim.api.nvim_win_close, s.win, true)
+  end
+  if vim.api.nvim_win_is_valid(s.caller_win) then
+    pcall(vim.api.nvim_set_current_win, s.caller_win)
+  end
+end
+
+local function _confirm()
+  if state == nil then
+    return
+  end
+  local line = vim.api.nvim_get_current_line()
+  local type = state.type
+  M._cleanup()
+  return line, type
+end
+
+--- Confirm and execute the current line as a cmdline.
+function M.confirm()
+  local line, type = _confirm()
+  vim.api.nvim_feedkeys(type .. line .. vim.keycode('<CR>'), 'nt', false)
+end
+
+--- Cancel: close the cmdwin and re-enter cmdline mode with the line pre-filled (no execute).
+function M.cancel()
+  local line, type = _confirm()
+  vim.api.nvim_feedkeys(type .. line, 'nt', false)
+end
+
+return M

runtime/lua/vim/_meta/api.gen.lua

@@ -24,6 +24,14 @@ function vim.api.nvim__buf_debug_extmarks(buf, keys, dot) end
 --- @return table<string,any>
 function vim.api.nvim__buf_stats(buf) end
 
+--- WARNING: This feature is experimental/unstable.
+---
+--- Records the cmdwin scratchbuf and type, or clears both when type="" / buf=0. Internal use only.
+---
+--- @param type string ':', '/', '?' (first char only); empty to clear.
+--- @param buf integer cmdwin buffer id, or 0 to clear.
+function vim.api.nvim__cmdwin_set(type, buf) end
+
 --- WARNING: This feature is experimental/unstable.
 ---
 --- Sets info for the completion item at the given index. If the info text was shown in a window,
@@ -967,12 +975,10 @@ function vim.api.nvim_create_autocmd(event, opts) end
 
 --- Creates a new, empty, unnamed buffer.
 ---
---- @see buf_open_scratch
 --- @param listed boolean Sets 'buflisted'
 --- @param scratch boolean Creates a "throwaway" `scratch-buffer` for temporary work
 --- (always 'nomodified'). Also sets 'nomodeline' on the buffer.
 --- @return integer # Buffer id, or 0 on error
----
 function vim.api.nvim_create_buf(listed, scratch) end
 
 --- Creates a new namespace or gets an existing one. [namespace]()