main: do not trigger FocusGained on startup

commit f045452743 introduced the
FocusGained autocommand. Unfortunately, for some systems this might
trigger already at startup, resulting in an unwanted redraw that might
clear the intro screen.

Therefore, add a timer, that enables the FocusGained autocommand after 5
seconds. The 5 seconds is somewhat arbritrarely, but we could change it
later if it turns out to be not useable.

fixes #1817

plugin/airline.vim

@@ -7,6 +7,7 @@ if &cp || v:version < 702 || (exists('g:loaded_airline') && g:loaded_airline)
   finish
 endif
 let g:loaded_airline = 1
+let s:has_timers = has("timers")
 
 let s:airline_initialized = 0
 function! s:init()
@@ -121,7 +122,16 @@ function! s:airline_toggle()
             \ |   call <sid>on_window_changed()
             \ | endif
 
-      autocmd VimResized,FocusGained * unlet! w:airline_lastmode | :call <sid>airline_refresh()
+      autocmd VimResized * unlet! w:airline_lastmode | :call <sid>airline_refresh()
+      if s:has_timers
+        " do not trigger FocusGained on startup, it might erase the intro
+        " screen (see #1817)
+        let Handler=funcref('<sid>FocusGainedHandler')
+        let s:timer=timer_start(5000, Handler)
+      else
+        autocmd FocusGained * unlet! w:airline_lastmode | :call <sid>airline_refresh()
+      endif
+
       autocmd TabEnter * :unlet! w:airline_lastmode | let w:airline_active=1
       autocmd BufWritePost */autoload/airline/themes/*.vim
             \ exec 'source '.split(globpath(&rtp, 'autoload/airline/themes/'.g:airline_theme.'.vim', 1), "\n")[0]
@@ -163,6 +173,14 @@ function! s:airline_refresh()
   call airline#update_statusline()
 endfunction
 
+function! s:FocusGainedHandler(timer)
+  if exists("s:timer") && a:timer == s:timer
+    augroup airline
+      au FocusGained * unlet! w:airline_lastmode | :call <sid>airline_refresh()
+    augroup END
+  endif
+endfu
+
 command! -bar -nargs=? -complete=customlist,<sid>get_airline_themes AirlineTheme call <sid>airline_theme(<f-args>)
 command! -bar AirlineToggleWhitespace call airline#extensions#whitespace#toggle()
 command! -bar AirlineToggle call s:airline_toggle()