#1700 - Try using a 0ms timer showing the completion menu

autoload/ale/completion.vim

@@ -1,17 +1,10 @@
 " Author: w0rp <devw0rp@gmail.com>
 " Description: Completion support for LSP linters
 
-function! ale#completion#TriggerOmnicompleteMenu() abort
-    " Replace completion options shortly before opening the menu.
-    call s:ReplaceCompletionOptions()
-
-    return "\<C-x>\<C-o>"
-endfunction
-
 " The omnicompletion menu is shown through a special Plug mapping which is
 " only valid in Insert mode. This way, feedkeys() won't send these keys if you
 " quit Insert mode quickly enough.
-inoremap <silent> <Plug>(ale_show_completion_menu) <C-R>=ale#completion#TriggerOmnicompleteMenu()<CR>
+inoremap <silent> <Plug>(ale_show_completion_menu) <C-x><C-o>
 " If we hit the key sequence in normal mode, then we won't show the menu, so
 " we should restore the old settings right away.
 nnoremap <silent> <Plug>(ale_show_completion_menu) :call ale#completion#RestoreCompletionOptions()<CR>
@@ -221,7 +214,10 @@ function! ale#completion#Show(response, completion_parser) abort
     " function, and then start omni-completion.
     let b:ale_completion_response = a:response
     let b:ale_completion_parser = a:completion_parser
-    call ale#util#FeedKeys("\<Plug>(ale_show_completion_menu)")
+    " Replace completion options shortly before opening the menu.
+    call s:ReplaceCompletionOptions()
+
+    call timer_start(0, {-> ale#util#FeedKeys("\<Plug>(ale_show_completion_menu)")})
 endfunction
 
 function! s:CompletionStillValid(request_id) abort

test/completion/test_completion_events.vader

@@ -125,32 +125,38 @@ Execute(ale#completion#Show() should remember the omnifunc setting and replace i
   let &l:omnifunc = 'FooBar'
 
   call ale#completion#Show('Response', 'Parser')
-  AssertEqual [["\<Plug>(ale_show_completion_menu)"]], g:feedkeys_calls
-  call ale#completion#TriggerOmnicompleteMenu()
 
   AssertEqual 'FooBar', b:ale_old_omnifunc
   AssertEqual 'ale#completion#OmniFunc', &l:omnifunc
 
+  AssertEqual [], g:feedkeys_calls
+  sleep 1ms
+  AssertEqual [["\<Plug>(ale_show_completion_menu)"]], g:feedkeys_calls
+
 Execute(ale#completion#Show() should remember the completeopt setting and replace it):
   let &l:completeopt = 'menu'
 
   call ale#completion#Show('Response', 'Parser')
-  AssertEqual [["\<Plug>(ale_show_completion_menu)"]], g:feedkeys_calls
-  call ale#completion#TriggerOmnicompleteMenu()
 
   AssertEqual 'menu', b:ale_old_completopt
   AssertEqual 'menu,menuone,noselect,noinsert', &l:completeopt
 
+  AssertEqual [], g:feedkeys_calls
+  sleep 1ms
+  AssertEqual [["\<Plug>(ale_show_completion_menu)"]], g:feedkeys_calls
+
 Execute(ale#completion#Show() should set the preview option if it's set):
   let &l:completeopt = 'menu,preview'
 
   call ale#completion#Show('Response', 'Parser')
-  AssertEqual [["\<Plug>(ale_show_completion_menu)"]], g:feedkeys_calls
-  call ale#completion#TriggerOmnicompleteMenu()
 
   AssertEqual 'menu,preview', b:ale_old_completopt
   AssertEqual 'menu,menuone,preview,noselect,noinsert', &l:completeopt
 
+  AssertEqual [], g:feedkeys_calls
+  sleep 1ms
+  AssertEqual [["\<Plug>(ale_show_completion_menu)"]], g:feedkeys_calls
+
 Execute(ale#completion#OmniFunc() should also remember the completeopt setting and replace it):
   let &l:completeopt = 'menu'
 
@@ -170,6 +176,8 @@ Execute(ale#completion#OmniFunc() should set the preview option if it's set):
 Execute(ale#completion#Show() should make the correct feedkeys() call):
   call ale#completion#Show('Response', 'Parser')
 
+  AssertEqual [], g:feedkeys_calls
+  sleep 1ms
   AssertEqual [["\<Plug>(ale_show_completion_menu)"]], g:feedkeys_calls
 
 Execute(ale#completion#Show() shouldn't do anything if you switch back to normal mode):