👍 Forcibly show info/warn/error messages block to the user

Users may not realize that there are info/warn/error messages in the
message history. This commit forcibly shows the messages to the user
when the messages are posted with `console.info/warn/error` functions
in Denops side.

Author: lambdalisue

autoload/denops/_internal/echo.vim

@@ -1,3 +1,8 @@
+const s:DELAYED_INTERVAL = 100
+
+let s:delayed_messages = []
+let s:delayed_timer = 0
+
 function! denops#_internal#echo#deprecate(...) abort
   if g:denops#disable_deprecation_warning_message
     return
@@ -17,15 +22,15 @@ function! denops#_internal#echo#debug(...) abort
 endfunction
 
 function! denops#_internal#echo#info(...) abort
-  call s:echomsg('Title', a:000)
+  call s:echomsg_delay('Title', a:000)
 endfunction
 
 function! denops#_internal#echo#warn(...) abort
-  call s:echomsg('WarningMsg', a:000)
+  call s:echomsg_delay('WarningMsg', a:000)
 endfunction
 
 function! denops#_internal#echo#error(...) abort
-  call s:echomsg('ErrorMsg', a:000)
+  call s:echomsg_delay('ErrorMsg', a:000)
 endfunction
 
 function! s:echomsg(hl, msg) abort
@@ -35,3 +40,21 @@ function! s:echomsg(hl, msg) abort
   endfor
   echohl None
 endfunction
+
+function! s:echomsg_delay(hl, msg) abort
+  call add(s:delayed_messages, [a:hl, a:msg])
+  call timer_stop(s:delayed_timer)
+  let s:delayed_timer = timer_start(s:DELAYED_INTERVAL, {-> s:echomsg_batch()})
+endfunction
+
+function! s:echomsg_batch() abort
+  let l:counter = 0
+  for l:message in s:delayed_messages
+    call s:echomsg(l:message[0], l:message[1])
+    let l:counter += len(split(join(l:message[1]), '\n'))
+  endfor
+  let s:delayed_timer = 0
+  let s:delayed_messages = []
+  " Forcibly show the messages to the user
+  call feedkeys(printf(":\<C-u>%dmessages\<CR>", l:counter), 'nt')
+endfunction