fix(api): nvim__complete_set requires completeopt=popup #31177

Problem:
If completeopt does not include "popup" flag, nvim__complete_set still
auto-creates a floating preview window.

Solution:
Fail if completeopt does not include the "popup" flag.

src/nvim/api/vim.c

@@ -44,6 +44,7 @@
 #include "nvim/highlight.h"
 #include "nvim/highlight_defs.h"
 #include "nvim/highlight_group.h"
+#include "nvim/insexpand.h"
 #include "nvim/keycodes.h"
 #include "nvim/log.h"
 #include "nvim/lua/executor.h"
@@ -2255,9 +2256,13 @@ void nvim_error_event(uint64_t channel_id, Integer lvl, String data)
 /// @return Dict containing these keys:
 ///       - winid: (number) floating window id
 ///       - bufnr: (number) buffer id in floating window
-Dict nvim__complete_set(Integer index, Dict(complete_set) *opts, Arena *arena)
+Dict nvim__complete_set(Integer index, Dict(complete_set) *opts, Arena *arena, Error *err)
 {
   Dict rv = arena_dict(arena, 2);
+  if ((get_cot_flags() & kOptCotFlagPopup) == 0) {
+    api_set_error(err, kErrorTypeException, "completeopt option does not include popup");
+    return rv;
+  }
   if (HAS_KEY(opts, complete_set, info)) {
     win_T *wp = pum_set_info((int)index, opts->info.data);
     if (wp) {

test/functional/editor/completion_spec.lua

@@ -10,6 +10,7 @@ local fn = n.fn
 local command = n.command
 local api = n.api
 local poke_eventloop = n.poke_eventloop
+local exec_lua = n.exec_lua
 
 describe('completion', function()
   local screen
@@ -1326,4 +1327,29 @@ describe('completion', function()
       ]],
     })
   end)
+
+  describe('nvim__complete_set', function()
+    it("fails when 'completeopt' does not include popup", function()
+      exec_lua([[
+        function _G.omni_test(findstart, base)
+          if findstart == 1 then
+            return vim.fn.col('.') - 1
+          end
+          return { { word = 'one' } }
+        end
+        vim.api.nvim_create_autocmd('CompleteChanged', {
+          callback = function()
+            local ok, err = pcall(vim.api.nvim__complete_set, 0, { info = '1info' })
+            if not ok then
+              vim.g.err_msg = err
+            end
+          end,
+        })
+        vim.opt.completeopt = 'menu,menuone'
+        vim.opt.omnifunc = 'v:lua.omni_test'
+      ]])
+      feed('S<C-X><C-O>')
+      eq('completeopt option does not include popup', api.nvim_get_var('err_msg'))
+    end)
+  end)
 end)